diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index ed4022d422b4d..04e56c373edc0 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -84,6 +84,7 @@ Other Enhancements
 - :meth:`Series.nlargest`, :meth:`Series.nsmallest`, :meth:`DataFrame.nlargest`, and :meth:`DataFrame.nsmallest` now accept the value ``"all"`` for the ``keep`` argument. This keeps all ties for the nth largest/smallest value (:issue:`16818`)
 - :class:`IntervalIndex` has gained the :meth:`~IntervalIndex.set_closed` method to change the existing ``closed`` value (:issue:`21670`)
 - :func:`~DataFrame.to_csv` and :func:`~DataFrame.to_json` now support ``compression='infer'`` to infer compression based on filename (:issue:`15008`)
+- :func:`to_timedelta` now supports iso-formated timedelta strings (:issue:`21877`)
 -
 
 .. _whatsnew_0240.api_breaking:
diff --git a/pandas/_libs/tslibs/timedeltas.pyx b/pandas/_libs/tslibs/timedeltas.pyx
index b9405b15a0980..f7a6cf0c6dafc 100644
--- a/pandas/_libs/tslibs/timedeltas.pyx
+++ b/pandas/_libs/tslibs/timedeltas.pyx
@@ -183,7 +183,11 @@ cpdef convert_to_timedelta64(object ts, object unit):
             ts = cast_from_unit(ts, unit)
             ts = np.timedelta64(ts)
     elif is_string_object(ts):
-        ts = np.timedelta64(parse_timedelta_string(ts))
+        if len(ts) > 0 and ts[0] == 'P':
+            ts = parse_iso_format_string(ts)
+        else:
+            ts = parse_timedelta_string(ts)
+        ts = np.timedelta64(ts)
     elif hasattr(ts, 'delta'):
         ts = np.timedelta64(delta_to_nanoseconds(ts), 'ns')
 
diff --git a/pandas/tests/indexes/timedeltas/test_construction.py b/pandas/tests/indexes/timedeltas/test_construction.py
index 68dc0003e2312..447e2b40050f6 100644
--- a/pandas/tests/indexes/timedeltas/test_construction.py
+++ b/pandas/tests/indexes/timedeltas/test_construction.py
@@ -44,6 +44,13 @@ def test_constructor(self):
         tm.assert_index_equal(TimedeltaIndex([400, 450, 1200], unit='ms'),
                               expected)
 
+    def test_constructor_iso(self):
+        # GH #21877
+        expected = timedelta_range('1s', periods=9, freq='s')
+        durations = ['P0DT0H0M{}S'.format(i) for i in range(1, 10)]
+        result = to_timedelta(durations)
+        tm.assert_index_equal(result, expected)
+
     def test_constructor_coverage(self):
         rng = timedelta_range('1 days', periods=10.5)
         exp = timedelta_range('1 days', periods=10)
diff --git a/pandas/tests/scalar/timedelta/test_timedelta.py b/pandas/tests/scalar/timedelta/test_timedelta.py
index 6472bd4245622..017606dc42d59 100644
--- a/pandas/tests/scalar/timedelta/test_timedelta.py
+++ b/pandas/tests/scalar/timedelta/test_timedelta.py
@@ -233,6 +233,11 @@ def check(value):
         assert tup.microseconds == 999
         assert tup.nanoseconds == 0
 
+    def test_iso_conversion(self):
+        # GH #21877
+        expected = Timedelta(1, unit='s')
+        assert to_timedelta('P0DT0H0M1S') == expected
+
     def test_nat_converters(self):
         assert to_timedelta('nat', box=False).astype('int64') == iNaT
         assert to_timedelta('nan', box=False).astype('int64') == iNaT