diff --git a/CHANGES.md b/CHANGES.md
index 658faad3a78..fd4d911287d 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -10,6 +10,9 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Fix a bug where an illegal trailing comma was added to return type annotations using
+  PEP 604 unions (#3735)
+
 ### Preview style
 
 <!-- Changes that affect Black's preview style -->
diff --git a/src/black/linegen.py b/src/black/linegen.py
index 0091cbb3bd1..ad21307c311 100644
--- a/src/black/linegen.py
+++ b/src/black/linegen.py
@@ -918,6 +918,13 @@ def bracket_split_build_line(
                     )
                     if isinstance(node, Node) and isinstance(node.prev_sibling, Leaf)
                 )
+                # Except the false negatives above for PEP 604 unions where we
+                # can't add the comma.
+                and not (
+                    leaves[0].parent
+                    and leaves[0].parent.next_sibling
+                    and leaves[0].parent.next_sibling.type == token.VBAR
+                )
             )
 
             if original.is_import or no_commas:
diff --git a/tests/data/simple_cases/pep_604.py b/tests/data/simple_cases/pep_604.py
new file mode 100644
index 00000000000..b68d59d6440
--- /dev/null
+++ b/tests/data/simple_cases/pep_604.py
@@ -0,0 +1,25 @@
+def some_very_long_name_function() -> my_module.Asdf | my_module.AnotherType | my_module.YetAnotherType | None:
+    pass
+
+
+def some_very_long_name_function() -> my_module.Asdf | my_module.AnotherType | my_module.YetAnotherType | my_module.EvenMoreType | None:
+    pass
+
+
+# output
+
+
+def some_very_long_name_function() -> (
+    my_module.Asdf | my_module.AnotherType | my_module.YetAnotherType | None
+):
+    pass
+
+
+def some_very_long_name_function() -> (
+    my_module.Asdf
+    | my_module.AnotherType
+    | my_module.YetAnotherType
+    | my_module.EvenMoreType
+    | None
+):
+    pass