diff --git a/doc/changes/dev/13109.bugfix.rst b/doc/changes/dev/13109.bugfix.rst
new file mode 100644
index 00000000000..00dc5c4dfb9
--- /dev/null
+++ b/doc/changes/dev/13109.bugfix.rst
@@ -0,0 +1 @@
+Fix reading annotations with :func:`mne.read_annotations` from .csv files containing nanoseconds in times, and drop nanoseconds from times returned from :meth:`mne.Annotations.to_data_frame` and saved in .csv files by :meth:`mne.Annotations.save`, by `Thomas Binns`_.
\ No newline at end of file
diff --git a/mne/annotations.py b/mne/annotations.py
index 74a5878dca2..e298e80918c 100644
--- a/mne/annotations.py
+++ b/mne/annotations.py
@@ -242,7 +242,8 @@ class Annotations:
         the annotations with raw data if their acquisition is started at the
         same time. If it is a string, it should conform to the ISO8601 format.
         More precisely to this '%%Y-%%m-%%d %%H:%%M:%%S.%%f' particular case of
-        the ISO8601 format where the delimiter between date and time is ' '.
+        the ISO8601 format where the delimiter between date and time is ' ' and at most
+        microsecond precision (nanoseconds are not supported).
     %(ch_names_annot)s
 
         .. versionadded:: 0.23
@@ -390,6 +391,20 @@ def __init__(
         extras=None,
     ):
         self._orig_time = _handle_meas_date(orig_time)
+        if isinstance(orig_time, str) and self._orig_time is None:
+            try:  # only warn if `orig_time` is not the default '1970-01-01 00:00:00'
+                if _handle_meas_date(0) == datetime.strptime(
+                    orig_time, "%Y-%m-%d %H:%M:%S"
+                ).replace(tzinfo=timezone.utc):
+                    pass
+            except ValueError:  # error if incorrect datetime format AND not the default
+                warn(
+                    "The format of the `orig_time` string is not recognised. It "
+                    "must conform to the ISO8601 format with at most microsecond "
+                    "precision and where the delimiter between date and time is "
+                    f"' '. Got: {orig_time}. Defaulting `orig_time` to None.",
+                    RuntimeWarning,
+                )
         self.onset, self.duration, self.description, self.ch_names, self._extras = (
             _check_o_d_s_c_e(onset, duration, description, ch_names, extras)
         )
@@ -615,7 +630,7 @@ def to_data_frame(self, time_format="datetime"):
             dt = _handle_meas_date(0)
         time_format = _check_time_format(time_format, valid_time_formats, dt)
         dt = dt.replace(tzinfo=None)
-        times = _convert_times(self.onset, time_format, dt)
+        times = _convert_times(self.onset, time_format, meas_date=dt, drop_nano=True)
         df = dict(onset=times, duration=self.duration, description=self.description)
         if self._any_ch_names():
             df.update(ch_names=self.ch_names)
@@ -1486,7 +1501,13 @@ def _read_annotations_csv(fname):
             "onsets in seconds."
         )
     except ValueError:
-        pass
+        # remove nanoseconds for ISO8601 (microsecond) compliance
+        timestamp = pd.Timestamp(orig_time)
+        timespec = "microseconds"
+        if timestamp == pd.Timestamp(_handle_meas_date(0)).astimezone(None):
+            timespec = "auto"  # use default timespec for `orig_time=None`
+        orig_time = timestamp.isoformat(sep=" ", timespec=timespec)
+
     onset_dt = pd.to_datetime(df["onset"])
     onset = (onset_dt - onset_dt[0]).dt.total_seconds()
     duration = df["duration"].values.astype(float)
diff --git a/mne/epochs.py b/mne/epochs.py
index d6159f49583..6c3935097bf 100644
--- a/mne/epochs.py
+++ b/mne/epochs.py
@@ -2861,7 +2861,7 @@ def to_data_frame(
         # prepare extra columns / multiindex
         mindex = list()
         times = np.tile(times, n_epochs)
-        times = _convert_times(times, time_format, self.info["meas_date"])
+        times = _convert_times(times, time_format, meas_date=self.info["meas_date"])
         mindex.append(("time", times))
         rev_event_id = {v: k for k, v in self.event_id.items()}
         conditions = [rev_event_id[k] for k in self.events[:, 2]]
diff --git a/mne/evoked.py b/mne/evoked.py
index 7bd2355e4ee..9a13f5229f6 100644
--- a/mne/evoked.py
+++ b/mne/evoked.py
@@ -1392,7 +1392,7 @@ def to_data_frame(
         data = _scale_dataframe_data(self, data, picks, scalings)
         # prepare extra columns / multiindex
         mindex = list()
-        times = _convert_times(times, time_format, self.info["meas_date"])
+        times = _convert_times(times, time_format, meas_date=self.info["meas_date"])
         mindex.append(("time", times))
         # build DataFrame
         df = _build_data_frame(
diff --git a/mne/io/base.py b/mne/io/base.py
index eb2b5bc49f3..314f981dcaf 100644
--- a/mne/io/base.py
+++ b/mne/io/base.py
@@ -2472,7 +2472,10 @@ def to_data_frame(
         # prepare extra columns / multiindex
         mindex = list()
         times = _convert_times(
-            times, time_format, self.info["meas_date"], self.first_time
+            times,
+            time_format,
+            meas_date=self.info["meas_date"],
+            first_time=self.first_time,
         )
         mindex.append(("time", times))
         # build DataFrame
diff --git a/mne/tests/test_annotations.py b/mne/tests/test_annotations.py
index 7a9a0faea43..596e37d5ce3 100644
--- a/mne/tests/test_annotations.py
+++ b/mne/tests/test_annotations.py
@@ -1138,6 +1138,35 @@ def test_broken_csv(tmp_path):
         read_annotations(fname)
 
 
+def test_nanosecond_in_times(tmp_path):
+    """Test onsets with ns read correctly for csv and caught as init argument."""
+    pd = pytest.importorskip("pandas")
+
+    # Test bad format onset sanitised when loading from csv
+    onset = (
+        pd.Timestamp(_ORIG_TIME)
+        .astimezone(None)
+        .isoformat(sep=" ", timespec="nanoseconds")
+    )
+    content = f"onset,duration,description\n{onset},1.0,AA"
+    fname = tmp_path / "annotations_broken.csv"
+    with open(fname, "w") as f:
+        f.write(content)
+    annot = read_annotations(fname)
+    assert annot.orig_time == _ORIG_TIME
+
+    # Test bad format `orig_time` str -> `None` raises warning in `Annotation` init
+    with pytest.warns(
+        RuntimeWarning, match="The format of the `orig_time` string is not recognised."
+    ):
+        bad_orig_time = (
+            pd.Timestamp(_ORIG_TIME)
+            .astimezone(None)
+            .isoformat(sep=" ", timespec="nanoseconds")
+        )
+        Annotations([0], [1], ["test"], bad_orig_time)
+
+
 # Test for IO with .txt files
 
 
@@ -1564,7 +1593,8 @@ def test_repr():
 @pytest.mark.parametrize("time_format", (None, "ms", "datetime", "timedelta"))
 def test_annotation_to_data_frame(time_format):
     """Test annotation class to data frame conversion."""
-    pytest.importorskip("pandas")
+    pd = pytest.importorskip("pandas")
+
     onset = np.arange(1, 10)
     durations = np.full_like(onset, [4, 5, 6, 4, 5, 6, 4, 5, 6])
     description = ["yy"] * onset.shape[0]
@@ -1584,6 +1614,12 @@ def test_annotation_to_data_frame(time_format):
     assert want == got
     assert df.groupby("description").count().onset["yy"] == 9
 
+    # Check nanoseconds omitted from onset times
+    if time_format == "datetime":
+        a.onset += 1e-7  # >6 decimals to trigger nanosecond component
+        df = a.to_data_frame(time_format=time_format)
+        assert pd.Timestamp(df.onset[0]).nanosecond == 0
+
 
 def test_annotation_ch_names():
     """Test annotation ch_names updating and pruning."""
diff --git a/mne/time_frequency/tfr.py b/mne/time_frequency/tfr.py
index 7f5632644be..3a69c93bc8d 100644
--- a/mne/time_frequency/tfr.py
+++ b/mne/time_frequency/tfr.py
@@ -2753,7 +2753,7 @@ def to_data_frame(
         # prepare extra columns / multiindex
         mindex = list()
         default_index = list()
-        times = _convert_times(times, time_format, self.info["meas_date"])
+        times = _convert_times(times, time_format, meas_date=self.info["meas_date"])
         times = np.tile(times, n_epochs * n_freqs * n_tapers)
         freqs = np.tile(np.repeat(freqs, n_times), n_epochs * n_tapers)
         mindex.append(("time", times))
diff --git a/mne/utils/dataframe.py b/mne/utils/dataframe.py
index 4b3bdb312bf..332caa5baeb 100644
--- a/mne/utils/dataframe.py
+++ b/mne/utils/dataframe.py
@@ -36,7 +36,9 @@ def _scale_dataframe_data(inst, data, picks, scalings):
     return data
 
 
-def _convert_times(times, time_format, meas_date=None, first_time=0):
+def _convert_times(
+    times, time_format, *, meas_date=None, first_time=0, drop_nano=False
+):
     """Convert vector of time in seconds to ms, datetime, or timedelta."""
     # private function; pandas already checked in calling function
     from pandas import to_timedelta
@@ -47,6 +49,11 @@ def _convert_times(times, time_format, meas_date=None, first_time=0):
         times = to_timedelta(times, unit="s")
     elif time_format == "datetime":
         times = to_timedelta(times + first_time, unit="s") + meas_date
+        if drop_nano:
+            tz_name = ""
+            if meas_date is not None and meas_date.tzinfo is not None:
+                tz_name = f", {meas_date.tzinfo.tzname(meas_date)}"  # timezone as str
+            times = times.astype(f"datetime64[us{tz_name}]")  # cap at microseconds
     return times