diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2230cbd49..21e9e8ae7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@
   - Deprecated `EventWaveform` neurodata type. @rly [#1940](https://github.com/NeurodataWithoutBorders/pynwb/pull/1940)
   - Deprecated `ImageMaskSeries` neurodata type. @rly [#1941](https://github.com/NeurodataWithoutBorders/pynwb/pull/1941)
 - Removed python 3.8 support, added python 3.13 support. @stephprince [#2007](https://github.com/NeurodataWithoutBorders/pynwb/pull/2007)
+- `mock_ElectricalSeries`. Make number of electrodes between data and electrode region agree when explicitly passing data @h-mayorquin [#2019](https://github.com/NeurodataWithoutBorders/pynwb/pull/2019)
 
 ### Documentation and tutorial enhancements
 - Updated `SpikeEventSeries`, `DecompositionSeries`, and `FilteredEphys` examples. @stephprince [#2012](https://github.com/NeurodataWithoutBorders/pynwb/pull/2012)
diff --git a/src/pynwb/testing/mock/ecephys.py b/src/pynwb/testing/mock/ecephys.py
index 0669e7493..315eb3d9c 100644
--- a/src/pynwb/testing/mock/ecephys.py
+++ b/src/pynwb/testing/mock/ecephys.py
@@ -83,15 +83,16 @@ def mock_ElectricalSeries(
     
     # Set a default rate if timestamps are not provided
     rate = 30_000.0 if (timestamps is None and rate is None) else rate
+    n_electrodes = data.shape[1] if data is not None else 5
 
     electrical_series = ElectricalSeries(
         name=name or name_generator("ElectricalSeries"),
         description=description,
-        data=data if data is not None else np.ones((10, 5)),
+        data=data if data is not None else np.ones((10, n_electrodes)),
         rate=rate,
         starting_time=starting_time,
         timestamps=timestamps,
-        electrodes=electrodes or mock_electrodes(nwbfile=nwbfile),
+        electrodes=electrodes or mock_electrodes(nwbfile=nwbfile, n_electrodes=n_electrodes),
         filtering=filtering,
         conversion=conversion,
         offset=offset,