diff --git a/doc/api/next_api_changes/deprecations/ 29529-TH.rst b/doc/api/next_api_changes/deprecations/ 29529-TH.rst
new file mode 100644
index 000000000000..e396e68c4aa1
--- /dev/null
+++ b/doc/api/next_api_changes/deprecations/ 29529-TH.rst	
@@ -0,0 +1,7 @@
+Capitalization of None in matplotlibrc
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In :file:`matplotlibrc` config files every capitalization of None was
+accepted for denoting the Python constant `None`. This is deprecated. The
+only accepted capitalization is now None, i.e. starting with a capital letter
+and all other letters in lowercase.
diff --git a/lib/matplotlib/rcsetup.py b/lib/matplotlib/rcsetup.py
index 04beb8126b74..e35517b07d19 100644
--- a/lib/matplotlib/rcsetup.py
+++ b/lib/matplotlib/rcsetup.py
@@ -191,6 +191,14 @@ def _make_type_validator(cls, *, allow_none=False):
     def validator(s):
         if (allow_none and
                 (s is None or cbook._str_lower_equal(s, "none"))):
+            if cbook._str_lower_equal(s, "none") and s != "None":
+                _api.warn_deprecated(
+                    "3.11",
+                    message=f"Using the capitalization {s!r} in matplotlibrc for "
+                            "*None* is deprecated in %(removal)s and will lead to an "
+                            "error from version 3.13 onward. Please use 'None' "
+                            "instead."
+                )
             return None
         if cls is str and not isinstance(s, str):
             raise ValueError(f'Could not convert {s!r} to str')