diff --git a/changelog/678.bugfix.rst b/changelog/678.bugfix.rst
new file mode 100644
index 0000000000..1ff55cfdb0
--- /dev/null
+++ b/changelog/678.bugfix.rst
@@ -0,0 +1 @@
+Make the :func:`.ext.commands.default_member_permissions` decorator always work in cogs.
diff --git a/disnake/ext/commands/base_core.py b/disnake/ext/commands/base_core.py
index 7a27ac2f9d..06c47ffd5f 100644
--- a/disnake/ext/commands/base_core.py
+++ b/disnake/ext/commands/base_core.py
@@ -223,6 +223,12 @@ def _ensure_assignment_on_copy(self, other: AppCommandT) -> AppCommandT:
             # _max_concurrency won't be None at this point
             other._max_concurrency = cast(MaxConcurrency, self._max_concurrency).copy()
 
+        if self.body._default_member_permissions != other.body._default_member_permissions and (
+            "default_member_permissions" not in other.__original_kwargs__
+            or self.body._default_member_permissions is not None
+        ):
+            other.body._default_member_permissions = self.body._default_member_permissions
+
         try:
             other.on_error = self.on_error
         except AttributeError:
diff --git a/disnake/ext/commands/ctx_menus_core.py b/disnake/ext/commands/ctx_menus_core.py
index 2ce13caecb..6b666072f5 100644
--- a/disnake/ext/commands/ctx_menus_core.py
+++ b/disnake/ext/commands/ctx_menus_core.py
@@ -205,15 +205,9 @@ def __init__(
         self.auto_sync: bool = True if auto_sync is None else auto_sync
 
         try:
-            default_perms: int = func.__default_member_permissions__
+            default_member_permissions = func.__default_member_permissions__
         except AttributeError:
             pass
-        else:
-            if default_member_permissions is not None:
-                raise ValueError(
-                    "Cannot set `default_member_permissions` in both parameter and decorator"
-                )
-            default_member_permissions = default_perms
 
         dm_permission = True if dm_permission is None else dm_permission
 
diff --git a/disnake/ext/commands/slash_core.py b/disnake/ext/commands/slash_core.py
index 5662f415e5..0bddf125c5 100644
--- a/disnake/ext/commands/slash_core.py
+++ b/disnake/ext/commands/slash_core.py
@@ -417,15 +417,9 @@ def __init__(
         desc_loc = Localized._cast(description, False)
 
         try:
-            default_perms: int = func.__default_member_permissions__
+            default_member_permissions = func.__default_member_permissions__
         except AttributeError:
             pass
-        else:
-            if default_member_permissions is not None:
-                raise ValueError(
-                    "Cannot set `default_member_permissions` in both parameter and decorator"
-                )
-            default_member_permissions = default_perms
 
         dm_permission = True if dm_permission is None else dm_permission
 
diff --git a/tests/ext/commands/test_base_core.py b/tests/ext/commands/test_base_core.py
new file mode 100644
index 0000000000..ce8293543d
--- /dev/null
+++ b/tests/ext/commands/test_base_core.py
@@ -0,0 +1,82 @@
+import pytest
+
+from disnake import Permissions
+from disnake.ext import commands
+
+
+class TestDefaultPermissions:
+    def test_decorator(self):
+        class Cog(commands.Cog):
+            @commands.slash_command(default_member_permissions=64)
+            async def cmd(self, _):
+                ...
+
+            @commands.default_member_permissions(64)
+            @commands.slash_command()
+            async def above(self, _):
+                ...
+
+            @commands.slash_command()
+            @commands.default_member_permissions(64)
+            async def below(self, _):
+                ...
+
+        for c in (Cog, Cog()):
+            assert c.cmd.default_member_permissions == Permissions(64)
+            assert c.above.default_member_permissions == Permissions(64)
+            assert c.below.default_member_permissions == Permissions(64)
+
+    def test_decorator_overwrite(self):
+        # putting the decorator above should fail
+        with pytest.raises(ValueError, match="Cannot set `default_member_permissions`"):
+
+            class Cog(commands.Cog):
+                @commands.default_member_permissions(32)
+                @commands.slash_command(default_member_permissions=64)
+                async def above(self, _):
+                    ...
+
+        # putting the decorator below shouldn't fail
+        # (this is a side effect of how command copying works,
+        # and while this *should* probably fail, we're just testing
+        # for regressions for now)
+        class Cog2(commands.Cog):
+            @commands.slash_command(default_member_permissions=64)
+            @commands.default_member_permissions(32)
+            async def below(self, _):
+                ...
+
+        for c in (Cog2, Cog2()):
+            assert c.below.default_member_permissions == Permissions(32)
+
+    def test_attrs(self):
+        class Cog(commands.Cog, slash_command_attrs={"default_member_permissions": 32}):
+            @commands.slash_command()
+            async def no_overwrite(self, _):
+                ...
+
+            @commands.slash_command(default_member_permissions=64)
+            async def overwrite(self, _):
+                ...
+
+            @commands.default_member_permissions(64)
+            @commands.slash_command()
+            async def overwrite_decorator_above(self, _):
+                ...
+
+            @commands.slash_command()
+            @commands.default_member_permissions(64)
+            async def overwrite_decorator_below(self, _):
+                ...
+
+        assert Cog.no_overwrite.default_member_permissions is None
+        assert Cog().no_overwrite.default_member_permissions == Permissions(32)
+
+        assert Cog.overwrite.default_member_permissions == Permissions(64)
+        assert Cog().overwrite.default_member_permissions == Permissions(64)
+
+        assert Cog.overwrite_decorator_above.default_member_permissions == Permissions(64)
+        assert Cog().overwrite_decorator_above.default_member_permissions == Permissions(64)
+
+        assert Cog.overwrite_decorator_below.default_member_permissions == Permissions(64)
+        assert Cog().overwrite_decorator_below.default_member_permissions == Permissions(64)