diff --git a/doc/changes/dev/13618.notable.rst b/doc/changes/dev/13618.notable.rst
new file mode 100644
index 00000000000..10202f8c55a
--- /dev/null
+++ b/doc/changes/dev/13618.notable.rst
@@ -0,0 +1 @@
+Clarify average EEG reference behavior in documentation for add_reference_channels.
\ No newline at end of file
diff --git a/mne/channels/channels.py b/mne/channels/channels.py
index c1a3d462525..c8ee5f74d57 100644
--- a/mne/channels/channels.py
+++ b/mne/channels/channels.py
@@ -812,6 +812,12 @@ def add_reference_channels(self, ref_channels):
         -------
         inst : same type as the input data
                The modified instance.
+
+        note:: If you are adding a new reference channel to data that
+                will eventually be used with an average reference,
+                you should also call :meth:`mne.io.Raw.set_eeg_reference`
+                (or the equivalent Epochs/Evoked method) to ensure the
+                mathematical reference is updated correctly.
         """
         return add_reference_channels(self, ref_channels, copy=False)
 
diff --git a/mne/utils/docs.py b/mne/utils/docs.py
index e079e2dd63c..0800e7bf33f 100644
--- a/mne/utils/docs.py
+++ b/mne/utils/docs.py
@@ -4106,6 +4106,11 @@ def _reflow_param_docstring(docstring, has_first_line=True, width=75):
     EEG signal by setting ``ref_channels='average'``. Bad EEG channels are
     automatically excluded if they are properly set in ``info['bads']``.
 
+    .. note::
+        If you wish to add a new reference channel (e.g., a mastoid)
+        to the data, use :func:`mne.add_reference_channels`
+        **before** calling this function.
+
 - A single electrode:
     Set ``ref_channels`` to a list containing the name of the channel that
     will act as the new reference, for example ``ref_channels=['Cz']``.
diff --git a/tutorials/preprocessing/55_setting_eeg_reference.py b/tutorials/preprocessing/55_setting_eeg_reference.py
index 770af624de7..813b38e603e 100644
--- a/tutorials/preprocessing/55_setting_eeg_reference.py
+++ b/tutorials/preprocessing/55_setting_eeg_reference.py
@@ -146,6 +146,11 @@
 #
 # Creating the average reference as a projector
 # ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+# .. note::
+#    If you wish to add a new reference channel (e.g., a mastoid)
+#    to the data, use :func:`mne.add_reference_channels`
+#    **before** calling this function.
 #
 # If using an average reference, it is possible to create the reference as a
 # :term:`projector` rather than subtracting the reference from the data