diff --git a/amaranth/lib/wiring.py b/amaranth/lib/wiring.py
index bc30bc8..bb8b4cb 100644
--- a/amaranth/lib/wiring.py
+++ b/amaranth/lib/wiring.py
@@ -353,6 +353,12 @@ class Signature(metaclass=SignatureMeta):
     def members(self):
         return self.__members
 
+    @members.setter
+    def members(self, new_members):
+        # The setter is called when `sig.members += ...` is used.
+        if new_members is not self.__members:
+            raise AttributeError("property 'members' of 'Signature' object cannot be set")
+
     def __eq__(self, other):
         other_unflipped = other.flip() if type(other) is FlippedSignature else other
         if type(self) is type(other_unflipped) is Signature:
@@ -518,6 +524,11 @@ class FlippedSignature:
     def members(self):
         return FlippedSignatureMembers(self.__unflipped.members)
 
+    @members.setter
+    def members(self, new_members):
+        if new_members.flip() is not self.__unflipped.members:
+            raise AttributeError("property 'members' of 'FlippedSignature' object cannot be set")
+
     def __eq__(self, other):
         if type(other) is FlippedSignature:
             # Trivial case.
@@ -550,10 +561,16 @@ class FlippedSignature:
             return getattr(self.__unflipped, name)
 
     def __setattr__(self, name, value):
-        try: # descriptor first
-            _gettypeattr(self.__unflipped, name).__set__(self, value)
-        except AttributeError:
-            setattr(self.__unflipped, name, value)
+        if name == "members":
+            # Although `sig.flip().members` does not call `__getattr__` but directly invokes
+            # the descriptor of the `FlippedSignature.members` property, `sig.flip().members +=`
+            # does call `__setattr__`, and this must be special-cased for the setter to work.
+            FlippedSignature.members.__set__(self, value)
+        else:
+            try: # descriptor first
+                _gettypeattr(self.__unflipped, name).__set__(self, value)
+            except AttributeError:
+                setattr(self.__unflipped, name, value)
 
     def __delattr__(self, name):
         try: # descriptor first
diff --git a/tests/test_lib_wiring.py b/tests/test_lib_wiring.py
index f5af0f2..c08f5dd 100644
--- a/tests/test_lib_wiring.py
+++ b/tests/test_lib_wiring.py
@@ -374,6 +374,18 @@ class SignatureTestCase(unittest.TestCase):
                 r"^Cannot add members to a frozen signature$"):
             sig.members += {"b": Out(1)}
 
+    def test_members_plus_equal(self):
+        sig = Signature({})
+        # This invokes the setter of Signature.members.
+        sig.members += {"a": In(1)}
+        self.assertIn("a", sig.members)
+
+    def test_members_equal_wrong(self):
+        sig = Signature({})
+        with self.assertRaisesRegex(AttributeError,
+                r"property 'members' of 'Signature' object cannot be set"):
+            sig.members = SignatureMembers({})
+
     def assertFlattenedSignature(self, actual, expected):
         for (a_path, a_member, a_value), (b_path, b_member, b_value) in zip(actual, expected):
             self.assertEqual(a_path, b_path)
@@ -595,6 +607,18 @@ class FlippedSignatureTestCase(unittest.TestCase):
         fsig.x()
         self.assertEqual(x_type, S)
 
+    def test_members_plus_equal(self):
+        sig = Signature({})
+        # This invokes the setter of FlippedSignature.members.
+        sig.flip().members += {"a": In(1)}
+        self.assertIn("a", sig.members)
+
+    def test_members_equal_wrong(self):
+        sig = Signature({})
+        with self.assertRaisesRegex(AttributeError,
+                r"property 'members' of 'FlippedSignature' object cannot be set"):
+            sig.flip().members = SignatureMembers({})
+
 
 class InterfaceTestCase(unittest.TestCase):
     pass