diff --git a/doc/source/whatsnew/v2.0.0.rst b/doc/source/whatsnew/v2.0.0.rst
index 45e4fd9f0aabb..35ace840c2cde 100644
--- a/doc/source/whatsnew/v2.0.0.rst
+++ b/doc/source/whatsnew/v2.0.0.rst
@@ -993,7 +993,7 @@ Period
 ^^^^^^
 - Bug in :meth:`Period.strftime` and :meth:`PeriodIndex.strftime`, raising ``UnicodeDecodeError`` when a locale-specific directive was passed (:issue:`46319`)
 - Bug in adding a :class:`Period` object to an array of :class:`DateOffset` objects incorrectly raising ``TypeError`` (:issue:`50162`)
--
+- Bug in :class:`Period` where passing a string with finer resolution than nanosecond would result in a ``KeyError`` instead of dropping the extra precision (:issue:`50417`)
 
 Plotting
 ^^^^^^^^
diff --git a/pandas/_libs/tslibs/parsing.pyx b/pandas/_libs/tslibs/parsing.pyx
index 0f6640a90d7ed..3de1345598437 100644
--- a/pandas/_libs/tslibs/parsing.pyx
+++ b/pandas/_libs/tslibs/parsing.pyx
@@ -393,7 +393,7 @@ cdef parse_datetime_string_with_reso(
         &out_tzoffset, False
     )
     if not string_to_dts_failed:
-        if dts.ps != 0 or out_local:
+        if out_bestunit == NPY_DATETIMEUNIT.NPY_FR_ns or out_local:
             # TODO: the not-out_local case we could do without Timestamp;
             #  avoid circular import
             from pandas import Timestamp
@@ -402,6 +402,13 @@ cdef parse_datetime_string_with_reso(
             parsed = datetime(
                 dts.year, dts.month, dts.day, dts.hour, dts.min, dts.sec, dts.us
             )
+        # Match Timestamp and drop picoseconds, femtoseconds, attoseconds
+        # The new resolution will just be nano
+        # GH 50417
+        if out_bestunit in {NPY_DATETIMEUNIT.NPY_FR_ps,
+                            NPY_DATETIMEUNIT.NPY_FR_fs,
+                            NPY_DATETIMEUNIT.NPY_FR_as}:
+            out_bestunit = NPY_DATETIMEUNIT.NPY_FR_ns
         reso = {
             NPY_DATETIMEUNIT.NPY_FR_Y: "year",
             NPY_DATETIMEUNIT.NPY_FR_M: "month",
diff --git a/pandas/tests/scalar/period/test_period.py b/pandas/tests/scalar/period/test_period.py
index 0636ecb023530..710a98ba6f005 100644
--- a/pandas/tests/scalar/period/test_period.py
+++ b/pandas/tests/scalar/period/test_period.py
@@ -545,6 +545,11 @@ def test_period_cons_combined(self):
             (".000000999", 999),
             (".123456789", 789),
             (".999999999", 999),
+            (".999999000", 0),
+            # Test femtoseconds, attoseconds, picoseconds are dropped like Timestamp
+            (".999999001123", 1),
+            (".999999001123456", 1),
+            (".999999001123456789", 1),
         ],
     )
     def test_period_constructor_nanosecond(self, day, hour, sec_float, expected):