diff --git a/doc/source/whatsnew/v1.5.0.rst b/doc/source/whatsnew/v1.5.0.rst
index e4879a6c41515..c418801c0a9f2 100644
--- a/doc/source/whatsnew/v1.5.0.rst
+++ b/doc/source/whatsnew/v1.5.0.rst
@@ -581,6 +581,8 @@ Period
 ^^^^^^
 - Bug in subtraction of :class:`Period` from :class:`PeriodArray` returning wrong results (:issue:`45999`)
 - Bug in :meth:`Period.strftime` and :meth:`PeriodIndex.strftime`, directives ``%l`` and ``%u`` were giving wrong results (:issue:`46252`)
+- Bug in inferring an incorrect ``freq`` when passing a string to :class:`Period` microseconds that are a multiple of 1000 (:issue:`46811`)
+- Bug in constructing a :class:`Period` from a :class:`Timestamp` or ``np.datetime64`` object with non-zero nanoseconds and ``freq="ns"`` incorrectly truncating the nanoseconds (:issue:`46811`)
 -
 
 Plotting
diff --git a/pandas/_libs/tslib.pyx b/pandas/_libs/tslib.pyx
index c96a65cdff525..0b35347fbd618 100644
--- a/pandas/_libs/tslib.pyx
+++ b/pandas/_libs/tslib.pyx
@@ -27,6 +27,7 @@ cnp.import_array()
 import pytz
 
 from pandas._libs.tslibs.np_datetime cimport (
+    NPY_DATETIMEUNIT,
     check_dts_bounds,
     dt64_to_dtstruct,
     dtstruct_to_dt64,
@@ -75,6 +76,7 @@ def _test_parse_iso8601(ts: str):
     cdef:
         _TSObject obj
         int out_local = 0, out_tzoffset = 0
+        NPY_DATETIMEUNIT out_bestunit
 
     obj = _TSObject()
 
@@ -83,7 +85,7 @@ def _test_parse_iso8601(ts: str):
     elif ts == 'today':
         return Timestamp.now().normalize()
 
-    string_to_dts(ts, &obj.dts, &out_local, &out_tzoffset, True)
+    string_to_dts(ts, &obj.dts, &out_bestunit, &out_local, &out_tzoffset, True)
     obj.value = dtstruct_to_dt64(&obj.dts)
     check_dts_bounds(&obj.dts)
     if out_local == 1:
@@ -428,6 +430,7 @@ cpdef array_to_datetime(
         ndarray[int64_t] iresult
         ndarray[object] oresult
         npy_datetimestruct dts
+        NPY_DATETIMEUNIT out_bestunit
         bint utc_convert = bool(utc)
         bint seen_integer = False
         bint seen_string = False
@@ -516,7 +519,7 @@ cpdef array_to_datetime(
                         continue
 
                     string_to_dts_failed = string_to_dts(
-                        val, &dts, &out_local,
+                        val, &dts, &out_bestunit, &out_local,
                         &out_tzoffset, False
                     )
                     if string_to_dts_failed:
diff --git a/pandas/_libs/tslibs/conversion.pyx b/pandas/_libs/tslibs/conversion.pyx
index ff54c399f6435..c876cc55be0be 100644
--- a/pandas/_libs/tslibs/conversion.pyx
+++ b/pandas/_libs/tslibs/conversion.pyx
@@ -586,6 +586,7 @@ cdef _TSObject _convert_str_to_tsobject(object ts, tzinfo tz, str unit,
         int out_local = 0, out_tzoffset = 0, string_to_dts_failed
         datetime dt
         int64_t ival
+        NPY_DATETIMEUNIT out_bestunit
 
     if len(ts) == 0 or ts in nat_strings:
         ts = NaT
@@ -604,7 +605,7 @@ cdef _TSObject _convert_str_to_tsobject(object ts, tzinfo tz, str unit,
         # equiv: datetime.today().replace(tzinfo=tz)
     else:
         string_to_dts_failed = string_to_dts(
-            ts, &dts, &out_local,
+            ts, &dts, &out_bestunit, &out_local,
             &out_tzoffset, False
         )
         if not string_to_dts_failed:
diff --git a/pandas/_libs/tslibs/np_datetime.pxd b/pandas/_libs/tslibs/np_datetime.pxd
index ecb318026f97b..f072dab3763aa 100644
--- a/pandas/_libs/tslibs/np_datetime.pxd
+++ b/pandas/_libs/tslibs/np_datetime.pxd
@@ -90,6 +90,7 @@ cdef NPY_DATETIMEUNIT get_datetime64_unit(object obj) nogil
 cdef int string_to_dts(
     str val,
     npy_datetimestruct* dts,
+    NPY_DATETIMEUNIT* out_bestunit,
     int* out_local,
     int* out_tzoffset,
     bint want_exc,
diff --git a/pandas/_libs/tslibs/np_datetime.pyx b/pandas/_libs/tslibs/np_datetime.pyx
index c611fec260ed0..07d198193464f 100644
--- a/pandas/_libs/tslibs/np_datetime.pyx
+++ b/pandas/_libs/tslibs/np_datetime.pyx
@@ -46,6 +46,7 @@ cdef extern from "src/datetime/np_datetime.h":
 cdef extern from "src/datetime/np_datetime_strings.h":
     int parse_iso_8601_datetime(const char *str, int len, int want_exc,
                                 npy_datetimestruct *out,
+                                NPY_DATETIMEUNIT *out_bestunit,
                                 int *out_local, int *out_tzoffset)
 
 
@@ -255,6 +256,7 @@ cdef inline int64_t pydate_to_dt64(date val, npy_datetimestruct *dts):
 cdef inline int string_to_dts(
     str val,
     npy_datetimestruct* dts,
+    NPY_DATETIMEUNIT* out_bestunit,
     int* out_local,
     int* out_tzoffset,
     bint want_exc,
@@ -265,7 +267,7 @@ cdef inline int string_to_dts(
 
     buf = get_c_string_buf_and_size(val, &length)
     return parse_iso_8601_datetime(buf, length, want_exc,
-                                   dts, out_local, out_tzoffset)
+                                   dts, out_bestunit, out_local, out_tzoffset)
 
 
 cpdef ndarray astype_overflowsafe(
diff --git a/pandas/_libs/tslibs/parsing.pyx b/pandas/_libs/tslibs/parsing.pyx
index 242a39b67fc44..8b42ed195957b 100644
--- a/pandas/_libs/tslibs/parsing.pyx
+++ b/pandas/_libs/tslibs/parsing.pyx
@@ -53,6 +53,11 @@ from pandas._libs.tslibs.nattype cimport (
     c_NaT as NaT,
     c_nat_strings as nat_strings,
 )
+from pandas._libs.tslibs.np_datetime cimport (
+    NPY_DATETIMEUNIT,
+    npy_datetimestruct,
+    string_to_dts,
+)
 from pandas._libs.tslibs.offsets cimport is_offset_object
 from pandas._libs.tslibs.util cimport (
     get_c_string_buf_and_size,
@@ -350,6 +355,11 @@ cdef parse_datetime_string_with_reso(
     """
     cdef:
         object parsed, reso
+        bint string_to_dts_failed
+        npy_datetimestruct dts
+        NPY_DATETIMEUNIT out_bestunit
+        int out_local
+        int out_tzoffset
 
     if not _does_string_look_like_datetime(date_string):
         raise ValueError('Given date string not likely a datetime.')
@@ -358,6 +368,33 @@ cdef parse_datetime_string_with_reso(
     if parsed is not None:
         return parsed, reso
 
+    # Try iso8601 first, as it handles nanoseconds
+    # TODO: does this render some/all of parse_delimited_date redundant?
+    string_to_dts_failed = string_to_dts(
+        date_string, &dts, &out_bestunit, &out_local,
+        &out_tzoffset, False
+    )
+    if not string_to_dts_failed:
+        if dts.ps != 0 or out_local:
+            # TODO: the not-out_local case we could do without Timestamp;
+            #  avoid circular import
+            from pandas import Timestamp
+            parsed = Timestamp(date_string)
+        else:
+            parsed = datetime(dts.year, dts.month, dts.day, dts.hour, dts.min, dts.sec, dts.us)
+        reso = {
+            NPY_DATETIMEUNIT.NPY_FR_Y: "year",
+            NPY_DATETIMEUNIT.NPY_FR_M: "month",
+            NPY_DATETIMEUNIT.NPY_FR_D: "day",
+            NPY_DATETIMEUNIT.NPY_FR_h: "hour",
+            NPY_DATETIMEUNIT.NPY_FR_m: "minute",
+            NPY_DATETIMEUNIT.NPY_FR_s: "second",
+            NPY_DATETIMEUNIT.NPY_FR_ms: "millisecond",
+            NPY_DATETIMEUNIT.NPY_FR_us: "microsecond",
+            NPY_DATETIMEUNIT.NPY_FR_ns: "nanosecond",
+            }[out_bestunit]
+        return parsed, reso
+
     try:
         return _parse_dateabbr_string(date_string, _DEFAULT_DATETIME, freq)
     except DateParseError:
diff --git a/pandas/_libs/tslibs/period.pyx b/pandas/_libs/tslibs/period.pyx
index 3d04562cb73c3..cfd358e61af9c 100644
--- a/pandas/_libs/tslibs/period.pyx
+++ b/pandas/_libs/tslibs/period.pyx
@@ -2584,10 +2584,13 @@ class Period(_Period):
             dt = value
             if freq is None:
                 raise ValueError('Must supply freq for datetime value')
+            if isinstance(dt, Timestamp):
+                nanosecond = dt.nanosecond
         elif util.is_datetime64_object(value):
             dt = Timestamp(value)
             if freq is None:
                 raise ValueError('Must supply freq for datetime value')
+            nanosecond = dt.nanosecond
         elif PyDate_Check(value):
             dt = datetime(year=value.year, month=value.month, day=value.day)
             if freq is None:
diff --git a/pandas/_libs/tslibs/src/datetime/np_datetime_strings.c b/pandas/_libs/tslibs/src/datetime/np_datetime_strings.c
index 847e84b21c06c..f787a26ab51fb 100644
--- a/pandas/_libs/tslibs/src/datetime/np_datetime_strings.c
+++ b/pandas/_libs/tslibs/src/datetime/np_datetime_strings.c
@@ -68,11 +68,13 @@ This file implements string parsing and creation for NumPy datetime.
  */
 int parse_iso_8601_datetime(const char *str, int len, int want_exc,
                             npy_datetimestruct *out,
+                            NPY_DATETIMEUNIT *out_bestunit,
                             int *out_local, int *out_tzoffset) {
     int year_leap = 0;
     int i, numdigits;
     const char *substr;
     int sublen;
+    NPY_DATETIMEUNIT bestunit = NPY_FR_GENERIC;
 
     /* If year-month-day are separated by a valid separator,
      * months/days without leading zeroes will be parsed
@@ -137,6 +139,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
         if (out_local != NULL) {
             *out_local = 0;
         }
+        bestunit = NPY_FR_Y;
         goto finish;
     }
 
@@ -182,6 +185,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
 
     /* Next character must be the separator, start of day, or end of string */
     if (sublen == 0) {
+        bestunit = NPY_FR_M;
         /* Forbid YYYYMM. Parsed instead as YYMMDD by someone else. */
         if (!has_ymd_sep) {
             goto parse_error;
@@ -231,6 +235,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
         if (out_local != NULL) {
             *out_local = 0;
         }
+        bestunit = NPY_FR_D;
         goto finish;
     }
 
@@ -269,6 +274,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
         if (!hour_was_2_digits) {
             goto parse_error;
         }
+        bestunit = NPY_FR_h;
         goto finish;
     }
 
@@ -310,6 +316,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
     }
 
     if (sublen == 0) {
+        bestunit = NPY_FR_m;
         goto finish;
     }
 
@@ -354,6 +361,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
         ++substr;
         --sublen;
     } else {
+        bestunit = NPY_FR_s;
         goto parse_timezone;
     }
 
@@ -370,6 +378,11 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
     }
 
     if (sublen == 0 || !isdigit(*substr)) {
+        if (numdigits > 3) {
+            bestunit = NPY_FR_us;
+        } else {
+            bestunit = NPY_FR_ms;
+        }
         goto parse_timezone;
     }
 
@@ -386,6 +399,11 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
     }
 
     if (sublen == 0 || !isdigit(*substr)) {
+        if (numdigits > 3) {
+            bestunit = NPY_FR_ps;
+        } else {
+            bestunit = NPY_FR_ns;
+        }
         goto parse_timezone;
     }
 
@@ -401,8 +419,14 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
         }
     }
 
+    if (numdigits > 3) {
+        bestunit = NPY_FR_as;
+    } else {
+        bestunit = NPY_FR_fs;
+    }
+
 parse_timezone:
-    /* trim any whitespace between time/timeezone */
+    /* trim any whitespace between time/timezone */
     while (sublen > 0 && isspace(*substr)) {
         ++substr;
         --sublen;
@@ -521,6 +545,9 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
     }
 
 finish:
+    if (out_bestunit != NULL) {
+        *out_bestunit = bestunit;
+    }
     return 0;
 
 parse_error:
diff --git a/pandas/_libs/tslibs/src/datetime/np_datetime_strings.h b/pandas/_libs/tslibs/src/datetime/np_datetime_strings.h
index 200a71ff0c2b7..2cc032c0e278c 100644
--- a/pandas/_libs/tslibs/src/datetime/np_datetime_strings.h
+++ b/pandas/_libs/tslibs/src/datetime/np_datetime_strings.h
@@ -56,6 +56,7 @@ This file implements string parsing and creation for NumPy datetime.
 int
 parse_iso_8601_datetime(const char *str, int len, int want_exc,
                         npy_datetimestruct *out,
+                        NPY_DATETIMEUNIT *out_bestunit,
                         int *out_local,
                         int *out_tzoffset);
 
diff --git a/pandas/tests/scalar/period/test_period.py b/pandas/tests/scalar/period/test_period.py
index 91578cbd7d614..f23d332c7e279 100644
--- a/pandas/tests/scalar/period/test_period.py
+++ b/pandas/tests/scalar/period/test_period.py
@@ -113,6 +113,21 @@ def test_construction(self):
         with pytest.raises(TypeError, match="pass as a string instead"):
             Period("1982", freq=("Min", 1))
 
+    def test_construction_from_timestamp_nanos(self):
+        # GH#46811 don't drop nanos from Timestamp
+        ts = Timestamp("2022-04-20 09:23:24.123456789")
+        per = Period(ts, freq="ns")
+
+        # should losslessly round-trip, not lose the 789
+        rt = per.to_timestamp()
+        assert rt == ts
+
+        # same thing but from a datetime64 object
+        dt64 = ts.asm8
+        per2 = Period(dt64, freq="ns")
+        rt2 = per2.to_timestamp()
+        assert rt2.asm8 == dt64
+
     def test_construction_bday(self):
 
         # Biz day construction, roll forward if non-weekday
@@ -324,8 +339,10 @@ def test_constructor_infer_freq(self):
         p = Period("2007-01-01 07:10:15.123")
         assert p.freq == "L"
 
+        # We see that there are 6 digits after the decimal, so get microsecond
+        #  even though they are all zeros.
         p = Period("2007-01-01 07:10:15.123000")
-        assert p.freq == "L"
+        assert p.freq == "U"
 
         p = Period("2007-01-01 07:10:15.123400")
         assert p.freq == "U"
diff --git a/pandas/tests/tslibs/test_parsing.py b/pandas/tests/tslibs/test_parsing.py
index 279a84b174e36..4dae6c586e306 100644
--- a/pandas/tests/tslibs/test_parsing.py
+++ b/pandas/tests/tslibs/test_parsing.py
@@ -23,6 +23,12 @@ def test_parse_time_string():
     assert parsed == parsed_lower
 
 
+def test_parse_time_string_nanosecond_reso():
+    # GH#46811
+    parsed, reso = parse_time_string("2022-04-20 09:19:19.123456789")
+    assert reso == "nanosecond"
+
+
 def test_parse_time_string_invalid_type():
     # Raise on invalid input, don't just return it
     msg = "Argument 'arg' has incorrect type (expected str, got tuple)"