diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst
index 2598ab40b23da77..59f41d137bc0a5f 100644
--- a/Doc/library/datetime.rst
+++ b/Doc/library/datetime.rst
@@ -1581,7 +1581,7 @@ Instance methods:
    and ``weekday``. The same as ``self.date().isocalendar()``.
 
 
-.. method:: datetime.isoformat(sep='T', timespec='auto')
+.. method:: datetime.isoformat(sep='T', timespec='auto', use_utc_designator=False)
 
    Return a string representing the date and time in ISO 8601 format:
 
@@ -1645,9 +1645,21 @@ Instance methods:
       >>> my_datetime.isoformat(timespec='microseconds')
       '2015-01-01T12:30:59.000000'
 
+   If the optional argument *use_utc_designator* is set to :const:`True` and
+   :meth:`tzname` returns exactly ``"UTC"``, then "Z" will be given as the UTC
+   offset in the formatted string::
+
+      >>> my_datetime = dt.datetime(2022, 3, 21, 12, 30, 59, tzinfo=dt.timezone.utc)
+      >>> my_datetime.isoformat()
+      '2022-03-21T12:30:59+00:00'
+      >>> my_datetime.isoformat(use_utc_designator=True)
+      '2022-03-21T12:30:59Z'
+
    .. versionchanged:: 3.6
       Added the *timespec* parameter.
 
+   .. versionadded:: 3.16
+      Added the *use_utc_designator* parameter.
 
 .. method:: datetime.__str__()
 
@@ -2003,7 +2015,7 @@ Instance methods:
       Added the *fold* parameter.
 
 
-.. method:: time.isoformat(timespec='auto')
+.. method:: time.isoformat(timespec='auto', use_utc_designator=False)
 
    Return a string representing the time in ISO 8601 format, one of:
 
@@ -2032,6 +2044,10 @@ Instance methods:
 
    :exc:`ValueError` will be raised on an invalid *timespec* argument.
 
+   If the optional argument *use_utc_designator* is set to :const:`True` and
+   :meth:`tzname` returns exactly ``"UTC"``, then "Z" will be given as the UTC
+   offset in the formatted string.
+
    Example::
 
       >>> import datetime as dt
@@ -2042,10 +2058,18 @@ Instance methods:
       '12:34:56.000000'
       >>> my_time.isoformat(timespec='auto')
       '12:34:56'
+      >>> my_time = dt.time(12, 30, 59, tzinfo=dt.timezone.utc)
+      >>> my_time.isoformat()
+      '12:30:59+00:00'
+      >>> my_time.isoformat(use_utc_designator=True)
+      '12:30:59Z'
 
    .. versionchanged:: 3.6
       Added the *timespec* parameter.
 
+   .. versionadded:: 3.16
+      Added the *use_utc_designator* parameter.
+
 
 .. method:: time.__str__()
 
diff --git a/Lib/_pydatetime.py b/Lib/_pydatetime.py
index f5ac965d3cfedf4..dd0ae20674a1e28 100644
--- a/Lib/_pydatetime.py
+++ b/Lib/_pydatetime.py
@@ -1626,11 +1626,13 @@ def __repr__(self):
             s = s[:-1] + ", fold=1)"
         return s
 
-    def isoformat(self, timespec='auto'):
+    def isoformat(self, timespec='auto', use_utc_designator=False):
         """Return the time formatted according to ISO.
 
         The full format is 'HH:MM:SS.mmmmmm+zz:zz'. By default, the fractional
-        part is omitted if self.microsecond == 0.
+        part is omitted if self.microsecond == 0. The UTC offset will be
+        replaced with a single "Z" if use_utc_designator is True and
+        self.tzname() is exactly "UTC"
 
         The optional argument timespec specifies the number of additional
         terms of the time to include. Valid options are 'auto', 'hours',
@@ -1638,9 +1640,12 @@ def isoformat(self, timespec='auto'):
         """
         s = _format_time(self._hour, self._minute, self._second,
                           self._microsecond, timespec)
-        tz = self._tzstr()
-        if tz:
-            s += tz
+        if use_utc_designator and "UTC" == self.tzname():
+            s += "Z"
+        else:
+            tz = self._tzstr()
+            if tz:
+                s += tz
         return s
 
     __str__ = isoformat
@@ -2175,14 +2180,16 @@ def ctime(self):
             self._hour, self._minute, self._second,
             self._year)
 
-    def isoformat(self, sep='T', timespec='auto'):
+    def isoformat(self, sep='T', timespec='auto', use_utc_designator=False):
         """Return the time formatted according to ISO.
 
         The full format looks like 'YYYY-MM-DD HH:MM:SS.mmmmmm'.
         By default, the fractional part is omitted if self.microsecond == 0.
 
-        If self.tzinfo is not None, the UTC offset is also attached, giving
-        a full format of 'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM'.
+        If self.tzinfo is not None, the UTC offset is also attached. If the
+        optional argument use_utc_designator is True and the timezone name is
+        "UTC", a Z is appended: 'YYYY-MM-DD HH:MM:SS.mmmmmmZ'.
+        Otherwise the format is 'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM'.
 
         Optional argument sep specifies the separator between date and
         time, default 'T'.
@@ -2196,9 +2203,12 @@ def isoformat(self, sep='T', timespec='auto'):
                           self._microsecond, timespec))
 
         off = self.utcoffset()
-        tz = _format_offset(off)
-        if tz:
-            s += tz
+        if use_utc_designator and "UTC" == self.tzname():
+            s += "Z"
+        else:
+            tz = _format_offset(off)
+            if tz:
+                s += tz
 
         return s
 
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index df7d540cb362f75..17cd0b15379221e 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -2304,6 +2304,8 @@ def test_isoformat(self):
         self.assertEqual(t.isoformat(timespec='microseconds'), "0001-02-03T04:05:01.000123")
         self.assertEqual(t.isoformat(timespec='auto'), "0001-02-03T04:05:01.000123")
         self.assertEqual(t.isoformat(sep=' ', timespec='minutes'), "0001-02-03 04:05")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "0001-02-03T04:05:01.000123")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "0001-02-03T04:05:01.000123")
         self.assertRaises(ValueError, t.isoformat, timespec='foo')
         # bpo-34482: Check that surrogates are handled properly.
         self.assertRaises(ValueError, t.isoformat, timespec='\ud800')
@@ -2312,6 +2314,8 @@ def test_isoformat(self):
 
         t = self.theclass(1, 2, 3, 4, 5, 1, 999500, tzinfo=timezone.utc)
         self.assertEqual(t.isoformat(timespec='milliseconds'), "0001-02-03T04:05:01.999+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "0001-02-03T04:05:01.999500+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "0001-02-03T04:05:01.999500Z")
 
         t = self.theclass(1, 2, 3, 4, 5, 1, 999500)
         self.assertEqual(t.isoformat(timespec='milliseconds'), "0001-02-03T04:05:01.999")
@@ -2331,6 +2335,8 @@ def test_isoformat(self):
         tz = FixedOffset(timedelta(seconds=16), 'XXX')
         t = self.theclass(2, 3, 2, tzinfo=tz)
         self.assertEqual(t.isoformat(), "0002-03-02T00:00:00+00:00:16")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "0002-03-02T00:00:00+00:00:16")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "0002-03-02T00:00:00+00:00:16")
 
     def test_isoformat_timezone(self):
         tzoffsets = [
@@ -4107,6 +4113,74 @@ def test_isoformat_timezone(self):
             with self.subTest(tzi=tzi):
                 self.assertEqual(t.isoformat(), exp)
 
+    def test_isoformat_utc_designator(self):
+        t = self.theclass(hour=12, minute=34, second=56, microsecond=123456)
+        self.assertEqual(t.isoformat(), "12:34:56.123456")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "12:34:56.123456")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "12:34:56.123456")
+
+        t = self.theclass(hour=12, minute=34, second=56, microsecond=123456,
+                          tzinfo=timezone.utc)
+        self.assertEqual(t.isoformat(), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "12:34:56.123456Z")
+
+        t = self.theclass(hour=12, minute=34, second=56, microsecond=123456,
+                          tzinfo=timezone(timedelta(0)))
+        self.assertEqual(t.isoformat(), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "12:34:56.123456Z")
+
+        t = self.theclass(hour=12, minute=34, second=56, microsecond=123456,
+                          tzinfo=timezone(timedelta(0), "UTC"))
+        self.assertEqual(t.isoformat(), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "12:34:56.123456Z")
+
+        t = self.theclass(hour=12, minute=34, second=56, microsecond=123456,
+                          tzinfo=timezone(timedelta(0), "GMT"))
+        self.assertEqual(t.isoformat(), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "12:34:56.123456+00:00")
+
+        t = self.theclass(hour=12, minute=34, second=56, microsecond=123456,
+                          tzinfo=timezone(timedelta(hours=5), "UTC"))
+        self.assertEqual(t.isoformat(), "12:34:56.123456+05:00")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "12:34:56.123456+05:00")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "12:34:56.123456Z")
+
+        class UnnamedTimezone(tzinfo):
+            def utcoffset(self, dt):
+                return timedelta(0)
+
+            def dst(self, dt):
+                return timedelta(0)
+
+            def tzname(self, dt):
+                return None
+
+        t = self.theclass(hour=12, minute=34, second=56, microsecond=123456,
+                          tzinfo=UnnamedTimezone())
+        self.assertEqual(t.isoformat(), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "12:34:56.123456+00:00")
+
+        class NonStringNamedTimezone(tzinfo):
+            def utcoffset(self, dt):
+                return timedelta(0)
+
+            def dst(self, dt):
+                return timedelta(0)
+
+            def tzname(self, dt):
+                return 42
+
+        t = self.theclass(hour=12, minute=34, second=56, microsecond=123456,
+                          tzinfo=UnnamedTimezone())
+        self.assertEqual(t.isoformat(), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=False), "12:34:56.123456+00:00")
+        self.assertEqual(t.isoformat(use_utc_designator=True), "12:34:56.123456+00:00")
+
     def test_1653736(self):
         # verify it doesn't accept extra keyword arguments
         t = self.theclass(second=1)
diff --git a/Misc/NEWS.d/next/Library/2022-03-22-00-27-11.bpo-46614.Rgt_1Y.rst b/Misc/NEWS.d/next/Library/2022-03-22-00-27-11.bpo-46614.Rgt_1Y.rst
new file mode 100644
index 000000000000000..eb5940805bb52ff
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-03-22-00-27-11.bpo-46614.Rgt_1Y.rst
@@ -0,0 +1,4 @@
+Add *use_utc_designator* as an optional parameter to
+:meth:`datetime.datetime.isoformat` and :meth:`datetime.time.isoformat`. If
+it's set to true, the UTC offset will be formatted as "Z" rather than "+00:00"
+if the object is associated with a timezone named exactly ``"UTC"``.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index cd2dd94c8164a53..eb27252cea9b9f5 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -1720,8 +1720,10 @@ format_ctime(PyObject *date, int hours, int minutes, int seconds)
 static PyObject *delta_negative(PyObject *op);
 
 /* Add formatted UTC offset string to buf.  buf has no more than
- * buflen bytes remaining.  The UTC offset is gotten by calling
- * tzinfo.uctoffset(tzinfoarg).  If that returns None, \0 is stored into
+ * buflen bytes remaining.  If use_utc_designator is true,
+ * tzinfo.tzname(tzinfoarg) will be called, and if it returns "UTC",
+ * only "Z\0" will be added. Otherwise, the UTC offset is gotten by calling
+ * tzinfo.utcoffset(tzinfoarg).  If that returns None, \0 is stored into
  * *buf, and that's all.  Else the returned value is checked for sanity (an
  * integer in range), and if that's OK it's converted to an hours & minutes
  * string of the form
@@ -1731,6 +1733,7 @@ static PyObject *delta_negative(PyObject *op);
  */
 static int
 format_utcoffset(char *buf, size_t buflen, const char *sep,
+                int use_utc_designator,
                 PyObject *tzinfo, PyObject *tzinfoarg)
 {
     PyObject *offset;
@@ -1739,6 +1742,20 @@ format_utcoffset(char *buf, size_t buflen, const char *sep,
 
     assert(buflen >= 1);
 
+    if (use_utc_designator) {
+        PyObject* name = PyObject_CallMethod(tzinfo, "tzname", "O", tzinfoarg);
+        if (name == NULL)
+            return -1;
+        int tz_is_utc = (PyUnicode_Check(name) &&
+                         0 == strcmp("UTC", PyUnicode_AsUTF8(name)));
+        Py_DECREF(name);
+
+        if (tz_is_utc) {
+            PyOS_snprintf(buf, buflen, "Z");
+            return 0;
+        }
+    }
+
     offset = call_utcoffset(tzinfo, tzinfoarg);
     if (offset == NULL)
         return -1;
@@ -1811,6 +1828,7 @@ make_somezreplacement(PyObject *object, char *sep, PyObject *tzinfoarg)
     if (format_utcoffset(buf,
                          sizeof(buf),
                          sep,
+                         0,
                          tzinfo,
                          tzinfoarg) < 0)
         return NULL;
@@ -4901,11 +4919,14 @@ time_str(PyObject *op)
 datetime.time.isoformat
 
     timespec: str(c_default="NULL") = 'auto'
+    use_utc_designator: bool = False
 
 Return the time formatted according to ISO.
 
 The full format is 'HH:MM:SS.mmmmmm+zz:zz'. By default, the
-fractional part is omitted if self.microsecond == 0.
+fractional part is omitted if self.microsecond == 0. The UTC offset
+will be replaced with a single "Z" if use_utc_designator is True and
+self.tzname() is exactly "UTC".
 
 The optional argument timespec specifies the number of additional
 terms of the time to include. Valid options are 'auto', 'hours',
@@ -4913,8 +4934,9 @@ terms of the time to include. Valid options are 'auto', 'hours',
 [clinic start generated code]*/
 
 static PyObject *
-datetime_time_isoformat_impl(PyDateTime_Time *self, const char *timespec)
-/*[clinic end generated code: output=2bcc7cab65c35545 input=0efae103081060f4]*/
+datetime_time_isoformat_impl(PyDateTime_Time *self, const char *timespec,
+                             int use_utc_designator)
+/*[clinic end generated code: output=938059b5f2a218ce input=6b40be753a404860]*/
 {
     char buf[100];
 
@@ -4965,8 +4987,8 @@ datetime_time_isoformat_impl(PyDateTime_Time *self, const char *timespec)
         return result;
 
     /* We need to append the UTC offset. */
-    if (format_utcoffset(buf, sizeof(buf), ":", self->tzinfo,
-                         Py_None) < 0) {
+    if (format_utcoffset(buf, sizeof(buf), ":", use_utc_designator,
+                         self->tzinfo, Py_None) < 0) {
         Py_DECREF(result);
         return NULL;
     }
@@ -6456,14 +6478,17 @@ datetime.datetime.isoformat
 
     sep: int(accept={str}) = 'T'
     timespec: str(c_default="NULL") = 'auto'
+    use_utc_designator: bool = False
 
 Return the time formatted according to ISO.
 
 The full format looks like 'YYYY-MM-DD HH:MM:SS.mmmmmm'.
 By default, the fractional part is omitted if self.microsecond == 0.
 
-If self.tzinfo is not None, the UTC offset is also attached, giving
-a full format of 'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM'.
+If self.tzinfo is not None, the UTC offset is also attached. If
+the optional argument use_utc_designator is True and the timezone
+name is "UTC", a Z is appended: 'YYYY-MM-DD HH:MM:SS.mmmmmmZ'.
+Otherwise the format is 'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM'.
 
 Optional argument sep specifies the separator between date and
 time, default 'T'.
@@ -6475,8 +6500,9 @@ terms of the time to include. Valid options are 'auto', 'hours',
 
 static PyObject *
 datetime_datetime_isoformat_impl(PyDateTime_DateTime *self, int sep,
-                                 const char *timespec)
-/*[clinic end generated code: output=9b6ce1383189b0bf input=db935a57fa697c5e]*/
+                                 const char *timespec,
+                                 int use_utc_designator)
+/*[clinic end generated code: output=1996e7d9ca2b38c8 input=0d182cca4b18668b]*/
 {
     char buffer[100];
 
@@ -6528,7 +6554,8 @@ datetime_datetime_isoformat_impl(PyDateTime_DateTime *self, int sep,
         return result;
 
     /* We need to append the UTC offset. */
-    if (format_utcoffset(buffer, sizeof(buffer), ":", self->tzinfo, (PyObject *)self) < 0) {
+    if (format_utcoffset(buffer, sizeof(buffer), ":", use_utc_designator,
+                         self->tzinfo, (PyObject *)self) < 0) {
         Py_DECREF(result);
         return NULL;
     }
diff --git a/Modules/clinic/_datetimemodule.c.h b/Modules/clinic/_datetimemodule.c.h
index 392a9da93ddbb41..050a10778894ebf 100644
--- a/Modules/clinic/_datetimemodule.c.h
+++ b/Modules/clinic/_datetimemodule.c.h
@@ -893,13 +893,15 @@ datetime_time_strptime(PyObject *type, PyObject *const *args, Py_ssize_t nargs)
 }
 
 PyDoc_STRVAR(datetime_time_isoformat__doc__,
-"isoformat($self, /, timespec=\'auto\')\n"
+"isoformat($self, /, timespec=\'auto\', use_utc_designator=False)\n"
 "--\n"
 "\n"
 "Return the time formatted according to ISO.\n"
 "\n"
 "The full format is \'HH:MM:SS.mmmmmm+zz:zz\'. By default, the\n"
-"fractional part is omitted if self.microsecond == 0.\n"
+"fractional part is omitted if self.microsecond == 0. The UTC offset\n"
+"will be replaced with a single \"Z\" if use_utc_designator is True and\n"
+"self.tzname() is exactly \"UTC\".\n"
 "\n"
 "The optional argument timespec specifies the number of additional\n"
 "terms of the time to include. Valid options are \'auto\', \'hours\',\n"
@@ -909,7 +911,8 @@ PyDoc_STRVAR(datetime_time_isoformat__doc__,
     {"isoformat", _PyCFunction_CAST(datetime_time_isoformat), METH_FASTCALL|METH_KEYWORDS, datetime_time_isoformat__doc__},
 
 static PyObject *
-datetime_time_isoformat_impl(PyDateTime_Time *self, const char *timespec);
+datetime_time_isoformat_impl(PyDateTime_Time *self, const char *timespec,
+                             int use_utc_designator);
 
 static PyObject *
 datetime_time_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
@@ -917,7 +920,7 @@ datetime_time_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
     PyObject *return_value = NULL;
     #if defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_MODULE)
 
-    #define NUM_KEYWORDS 1
+    #define NUM_KEYWORDS 2
     static struct {
         PyGC_Head _this_is_not_used;
         PyObject_VAR_HEAD
@@ -926,7 +929,7 @@ datetime_time_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
     } _kwtuple = {
         .ob_base = PyVarObject_HEAD_INIT(&PyTuple_Type, NUM_KEYWORDS)
         .ob_hash = -1,
-        .ob_item = { &_Py_ID(timespec), },
+        .ob_item = { &_Py_ID(timespec), &_Py_ID(use_utc_designator), },
     };
     #undef NUM_KEYWORDS
     #define KWTUPLE (&_kwtuple.ob_base.ob_base)
@@ -935,35 +938,45 @@ datetime_time_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
     #  define KWTUPLE NULL
     #endif  // !Py_BUILD_CORE
 
-    static const char * const _keywords[] = {"timespec", NULL};
+    static const char * const _keywords[] = {"timespec", "use_utc_designator", NULL};
     static _PyArg_Parser _parser = {
         .keywords = _keywords,
         .fname = "isoformat",
         .kwtuple = KWTUPLE,
     };
     #undef KWTUPLE
-    PyObject *argsbuf[1];
+    PyObject *argsbuf[2];
     Py_ssize_t noptargs = nargs + (kwnames ? PyTuple_GET_SIZE(kwnames) : 0) - 0;
     const char *timespec = NULL;
+    int use_utc_designator = 0;
 
     args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser,
-            /*minpos*/ 0, /*maxpos*/ 1, /*minkw*/ 0, /*varpos*/ 0, argsbuf);
+            /*minpos*/ 0, /*maxpos*/ 2, /*minkw*/ 0, /*varpos*/ 0, argsbuf);
     if (!args) {
         goto exit;
     }
     if (!noptargs) {
         goto skip_optional_pos;
     }
-    if (!PyUnicode_Check(args[0])) {
-        _PyArg_BadArgument("isoformat", "argument 'timespec'", "str", args[0]);
-        goto exit;
+    if (args[0]) {
+        if (!PyUnicode_Check(args[0])) {
+            _PyArg_BadArgument("isoformat", "argument 'timespec'", "str", args[0]);
+            goto exit;
+        }
+        timespec = _PyUnicode_AsUTF8NoNUL(args[0]);
+        if (timespec == NULL) {
+            goto exit;
+        }
+        if (!--noptargs) {
+            goto skip_optional_pos;
+        }
     }
-    timespec = _PyUnicode_AsUTF8NoNUL(args[0]);
-    if (timespec == NULL) {
+    use_utc_designator = PyObject_IsTrue(args[1]);
+    if (use_utc_designator < 0) {
         goto exit;
     }
 skip_optional_pos:
-    return_value = datetime_time_isoformat_impl((PyDateTime_Time *)self, timespec);
+    return_value = datetime_time_isoformat_impl((PyDateTime_Time *)self, timespec, use_utc_designator);
 
 exit:
     return return_value;
@@ -1722,7 +1735,7 @@ datetime_datetime_fromisoformat(PyObject *type, PyObject *arg)
 }
 
 PyDoc_STRVAR(datetime_datetime_isoformat__doc__,
-"isoformat($self, /, sep=\'T\', timespec=\'auto\')\n"
+"isoformat($self, /, sep=\'T\', timespec=\'auto\', use_utc_designator=False)\n"
 "--\n"
 "\n"
 "Return the time formatted according to ISO.\n"
@@ -1730,8 +1743,10 @@ PyDoc_STRVAR(datetime_datetime_isoformat__doc__,
 "The full format looks like \'YYYY-MM-DD HH:MM:SS.mmmmmm\'.\n"
 "By default, the fractional part is omitted if self.microsecond == 0.\n"
 "\n"
-"If self.tzinfo is not None, the UTC offset is also attached, giving\n"
-"a full format of \'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM\'.\n"
+"If self.tzinfo is not None, the UTC offset is also attached. If\n"
+"the optional argument use_utc_designator is True and the timezone\n"
+"name is \"UTC\", a Z is appended: \'YYYY-MM-DD HH:MM:SS.mmmmmmZ\'.\n"
+"Otherwise the format is \'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM\'.\n"
 "\n"
 "Optional argument sep specifies the separator between date and\n"
 "time, default \'T\'.\n"
@@ -1745,7 +1760,8 @@ PyDoc_STRVAR(datetime_datetime_isoformat__doc__,
 
 static PyObject *
 datetime_datetime_isoformat_impl(PyDateTime_DateTime *self, int sep,
-                                 const char *timespec);
+                                 const char *timespec,
+                                 int use_utc_designator);
 
 static PyObject *
 datetime_datetime_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
@@ -1753,7 +1769,7 @@ datetime_datetime_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t na
     PyObject *return_value = NULL;
     #if defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_MODULE)
 
-    #define NUM_KEYWORDS 2
+    #define NUM_KEYWORDS 3
     static struct {
         PyGC_Head _this_is_not_used;
         PyObject_VAR_HEAD
@@ -1762,7 +1778,7 @@ datetime_datetime_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t na
     } _kwtuple = {
         .ob_base = PyVarObject_HEAD_INIT(&PyTuple_Type, NUM_KEYWORDS)
         .ob_hash = -1,
-        .ob_item = { &_Py_ID(sep), &_Py_ID(timespec), },
+        .ob_item = { &_Py_ID(sep), &_Py_ID(timespec), &_Py_ID(use_utc_designator), },
     };
     #undef NUM_KEYWORDS
     #define KWTUPLE (&_kwtuple.ob_base.ob_base)
@@ -1771,20 +1787,21 @@ datetime_datetime_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t na
     #  define KWTUPLE NULL
     #endif  // !Py_BUILD_CORE
 
-    static const char * const _keywords[] = {"sep", "timespec", NULL};
+    static const char * const _keywords[] = {"sep", "timespec", "use_utc_designator", NULL};
     static _PyArg_Parser _parser = {
         .keywords = _keywords,
         .fname = "isoformat",
         .kwtuple = KWTUPLE,
     };
     #undef KWTUPLE
-    PyObject *argsbuf[2];
+    PyObject *argsbuf[3];
     Py_ssize_t noptargs = nargs + (kwnames ? PyTuple_GET_SIZE(kwnames) : 0) - 0;
     int sep = 'T';
     const char *timespec = NULL;
+    int use_utc_designator = 0;
 
     args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser,
-            /*minpos*/ 0, /*maxpos*/ 2, /*minkw*/ 0, /*varpos*/ 0, argsbuf);
+            /*minpos*/ 0, /*maxpos*/ 3, /*minkw*/ 0, /*varpos*/ 0, argsbuf);
     if (!args) {
         goto exit;
     }
@@ -1808,16 +1825,25 @@ datetime_datetime_isoformat(PyObject *self, PyObject *const *args, Py_ssize_t na
             goto skip_optional_pos;
         }
     }
-    if (!PyUnicode_Check(args[1])) {
-        _PyArg_BadArgument("isoformat", "argument 'timespec'", "str", args[1]);
-        goto exit;
+    if (args[1]) {
+        if (!PyUnicode_Check(args[1])) {
+            _PyArg_BadArgument("isoformat", "argument 'timespec'", "str", args[1]);
+            goto exit;
+        }
+        timespec = _PyUnicode_AsUTF8NoNUL(args[1]);
+        if (timespec == NULL) {
+            goto exit;
+        }
+        if (!--noptargs) {
+            goto skip_optional_pos;
+        }
     }
-    timespec = _PyUnicode_AsUTF8NoNUL(args[1]);
-    if (timespec == NULL) {
+    use_utc_designator = PyObject_IsTrue(args[2]);
+    if (use_utc_designator < 0) {
         goto exit;
     }
 skip_optional_pos:
-    return_value = datetime_datetime_isoformat_impl((PyDateTime_DateTime *)self, sep, timespec);
+    return_value = datetime_datetime_isoformat_impl((PyDateTime_DateTime *)self, sep, timespec, use_utc_designator);
 
 exit:
     return return_value;
@@ -2082,4 +2108,4 @@ datetime_datetime___reduce__(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
     return datetime_datetime___reduce___impl((PyDateTime_DateTime *)self);
 }
-/*[clinic end generated code: output=7bdfdfbd568c63ed input=a9049054013a1b77]*/
+/*[clinic end generated code: output=3416cb44b54c6220 input=a9049054013a1b77]*/