diff --git a/ChangeLog b/ChangeLog
index d730e8e829..bacf9baa64 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -41,6 +41,10 @@ Release date: TBA
 
   Closes #2684
 
+* Fix bug where ``pylint code.custom_extension`` would analyze ``code.py`` or ``code.pyi`` instead if they existed.
+
+	Closes pylint-dev/pylint#3631
+
 
 What's New in astroid 3.3.9?
 ============================
diff --git a/astroid/modutils.py b/astroid/modutils.py
index 29d09f860a..6029e33c1d 100644
--- a/astroid/modutils.py
+++ b/astroid/modutils.py
@@ -489,8 +489,9 @@ def get_source_file(
     """
     filename = os.path.abspath(_path_from_filename(filename))
     base, orig_ext = os.path.splitext(filename)
-    if orig_ext == ".pyi" and os.path.exists(f"{base}{orig_ext}"):
-        return f"{base}{orig_ext}"
+    orig_ext = orig_ext.lstrip(".")
+    if orig_ext not in PY_SOURCE_EXTS and os.path.exists(f"{base}.{orig_ext}"):
+        return f"{base}.{orig_ext}"
     for ext in PY_SOURCE_EXTS_STUBS_FIRST if prefer_stubs else PY_SOURCE_EXTS:
         source_path = f"{base}.{ext}"
         if os.path.exists(source_path):
diff --git a/script/.contributors_aliases.json b/script/.contributors_aliases.json
index 7bf909b149..1289a9a9be 100644
--- a/script/.contributors_aliases.json
+++ b/script/.contributors_aliases.json
@@ -1,4 +1,8 @@
 {
+  "c.ringstrom@gmail.com": {
+    "mails": ["c.ringstrom@gmail.com"],
+    "name": "Charlie Ringström"
+  },
   "134317971+correctmost@users.noreply.github.com": {
     "mails": ["134317971+correctmost@users.noreply.github.com"],
     "name": "correctmost"
diff --git a/tests/test_modutils.py b/tests/test_modutils.py
index e1b4be3841..cdd677edd9 100644
--- a/tests/test_modutils.py
+++ b/tests/test_modutils.py
@@ -335,6 +335,22 @@ def test_pyi_preferred(self) -> None:
             os.path.normpath(module) + "i",
         )
 
+    def test_nonstandard_extension(self) -> None:
+        package = resources.find("pyi_data/find_test")
+        modules = [
+            os.path.join(package, "__init__.weird_ext"),
+            os.path.join(package, "standalone_file.weird_ext"),
+        ]
+        for module in modules:
+            self.assertEqual(
+                modutils.get_source_file(module, prefer_stubs=True),
+                module,
+            )
+            self.assertEqual(
+                modutils.get_source_file(module),
+                module,
+            )
+
 
 class IsStandardModuleTest(resources.SysPathSetup, unittest.TestCase):
     """
diff --git a/tests/testdata/python3/pyi_data/find_test/__init__.weird_ext b/tests/testdata/python3/pyi_data/find_test/__init__.weird_ext
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/tests/testdata/python3/pyi_data/find_test/standalone_file.weird_ext b/tests/testdata/python3/pyi_data/find_test/standalone_file.weird_ext
new file mode 100644
index 0000000000..e69de29bb2