diff --git a/shiny/_modules.py b/shiny/_modules.py
index cd5151bea..89aa96647 100644
--- a/shiny/_modules.py
+++ b/shiny/_modules.py
@@ -1,5 +1,6 @@
 __all__ = ("namespaced_id", "module_ui", "module_server")
 
+import copy
 import sys
 from typing import Any, Callable, TypeVar
 
@@ -26,7 +27,8 @@ class ModuleInputs(Inputs):
     """
 
     def __init__(self, ns: str, parent_inputs: Inputs):
-        self._ns = namespaced_id(ns, parent_inputs._ns)  # Support nested modules
+        self._namespaces = copy.copy(parent_inputs._namespaces)
+        self._namespaces.insert(0, ns)
         # Don't set _parent attribute like the other classes since Inputs redefines
         # __setattr__
         self._map = parent_inputs._map
@@ -46,7 +48,8 @@ class ModuleOutputs(Outputs):
     """
 
     def __init__(self, ns: str, parent_outputs: Outputs):
-        self._ns = namespaced_id(ns, parent_outputs._ns)  # Support nested modules
+        self._namespaces = copy.copy(parent_outputs._namespaces)
+        self._namespaces.insert(0, ns)
         self._parent = parent_outputs
 
     def __getattr__(self, attr: str) -> Any:
@@ -67,7 +70,8 @@ class ModuleSession(Session):
     """
 
     def __init__(self, ns: str, parent_session: Session):
-        self._ns: str = namespaced_id(ns, parent_session._ns)  # Support nested modules
+        self._namespaces = copy.copy(parent_session._namespaces)
+        self._namespaces.append(ns)
         self._parent: Session = parent_session
         self.input: ModuleInputs = ModuleInputs(ns, parent_session.input)
         self.output: ModuleOutputs = ModuleOutputs(ns, parent_session.output)
@@ -78,7 +82,7 @@ def __getattr__(self, attr: str) -> Any:
 
 class MockModuleSession(ModuleSession):
     def __init__(self, ns: str):
-        self._ns = ns
+        self._namespaces = [ns]
 
 
 P = ParamSpec("P")
@@ -87,6 +91,7 @@ def __init__(self, ns: str):
 
 def module_ui(fn: Callable[P, R]) -> Callable[Concatenate[str, P], R]:
     def wrapper(ns: str, *args: P.args, **kwargs: P.kwargs) -> R:
+        # TODO: what should happen if this is called *inside* of a session? Do we pass in the parent session?
         with session_context(MockModuleSession(ns)):
             return fn(*args, **kwargs)
 
diff --git a/shiny/_namespaces.py b/shiny/_namespaces.py
index d5d7b736b..005635f69 100644
--- a/shiny/_namespaces.py
+++ b/shiny/_namespaces.py
@@ -1,34 +1,31 @@
 __all__ = ("namespaced_id",)
 
-from typing import Union, Optional
+from typing import Union, List
 
-from .types import MISSING, MISSING_TYPE
 
+class ResolvedId(str):
+    pass
 
-def namespaced_id(id: str, ns: Union[str, MISSING_TYPE, None] = MISSING) -> str:
-    """
-    Namespace an ID based on the current :func:`module_ui`/:func:`module_server`
-    namespace.
 
-    Parameters
-    ----------
-    id
-        The ID to namespace..
-    """
-    if isinstance(ns, MISSING_TYPE):
-        ns = get_current_namespace()
+Id = Union[str, ResolvedId]
 
-    if ns is None:
+
+def namespaced_id(id: str) -> str:
+    return namespaced_id_ns(id, get_current_namespaces())
+
+
+def namespaced_id_ns(id: Id, namespaces: List[str] = []) -> str:
+    if isinstance(id, ResolvedId) or len(namespaces) == 0:
         return id
     else:
-        return ns + "_" + id
+        return ResolvedId("_".join(namespaces) + "_" + id)
 
 
-def get_current_namespace() -> Optional[str]:
+def get_current_namespaces() -> List[str]:
     from .session import get_current_session
 
     session = get_current_session()
     if session is None:
-        return None
+        return []
     else:
-        return session._ns
+        return session._namespaces
diff --git a/shiny/session/_session.py b/shiny/session/_session.py
index 4002f01db..7b0d5d8e9 100644
--- a/shiny/session/_session.py
+++ b/shiny/session/_session.py
@@ -50,7 +50,7 @@
 from .._fileupload import FileInfo, FileUploadManager
 from ..http_staticfiles import FileResponse
 from ..input_handler import input_handlers
-from .._namespaces import namespaced_id
+from .._namespaces import namespaced_id_ns
 from ..reactive import Value, Effect, Effect_, isolate, flush
 from ..reactive._core import lock
 from ..types import SafeException, SilentCancelOutputException, SilentException
@@ -143,7 +143,7 @@ def __init__(
         self.input: Inputs = Inputs()
         self.output: Outputs = Outputs(self)
 
-        self._ns: Optional[str] = None  # Only relevant for ModuleSession
+        self._namespaces: List[str] = []  # i.e., ModuleSession namespaces
 
         self.user: Union[str, None] = None
         self.groups: Union[List[str], None] = None
@@ -479,7 +479,10 @@ def send_input_message(self, id: str, message: Dict[str, object]) -> None:
         message
             The message to send.
         """
-        msg: Dict[str, object] = {"id": namespaced_id(id, self._ns), "message": message}
+        msg: Dict[str, object] = {
+            "id": namespaced_id_ns(id, self._namespaces),
+            "message": message,
+        }
         self._outbound_message_queues["input_messages"].append(msg)
         self._request_flush()
 
@@ -744,16 +747,16 @@ def __init__(self, **kwargs: object) -> None:
         for key, value in kwargs.items():
             self._map[key] = Value(value, read_only=True)
 
-        self._ns: Optional[str] = None  # Only relevant for ModuleInputs()
+        self._namespaces: List[str] = []  # i.e. ModuleInputs() namespaces
 
     def __setitem__(self, key: str, value: Value[Any]) -> None:
         if not isinstance(value, Value):
             raise TypeError("`value` must be a reactive.Value object.")
 
-        self._map[namespaced_id(key, self._ns)] = value
+        self._map[namespaced_id_ns(key, self._namespaces)] = value
 
     def __getitem__(self, key: str) -> Value[Any]:
-        key = namespaced_id(key, self._ns)
+        key = namespaced_id_ns(key, self._namespaces)
         # Auto-populate key if accessed but not yet set. Needed to take reactive
         # dependencies on input values that haven't been received from client
         # yet.
@@ -763,18 +766,18 @@ def __getitem__(self, key: str) -> Value[Any]:
         return self._map[key]
 
     def __delitem__(self, key: str) -> None:
-        del self._map[namespaced_id(key, self._ns)]
+        del self._map[namespaced_id_ns(key, self._namespaces)]
 
     # Allow access of values as attributes.
     def __setattr__(self, attr: str, value: Value[Any]) -> None:
-        if attr in ("_map", "_ns"):
+        if attr in ("_map", "_namespaces"):
             super().__setattr__(attr, value)
             return
 
         self.__setitem__(attr, value)
 
     def __getattr__(self, attr: str) -> Value[Any]:
-        if attr in ("_map", "_ns"):
+        if attr in ("_map", "_namespaces"):
             return object.__getattribute__(self, attr)
         return self.__getitem__(attr)
 
@@ -801,7 +804,7 @@ def __init__(self, session: Session) -> None:
         self._effects: Dict[str, Effect_] = {}
         self._suspend_when_hidden: Dict[str, bool] = {}
         self._session: Session = session
-        self._ns: Optional[str] = None  # Only relevant for ModuleOutputs()
+        self._namespaces: List[str] = []  # i.e., ModuleOutputs() namespaces
 
     def __call__(
         self,
@@ -812,7 +815,7 @@ def __call__(
     ) -> Callable[[render.RenderFunction], None]:
         def set_fn(fn: render.RenderFunction) -> None:
             # Get the (possibly namespaced) output id
-            fn_name = namespaced_id(name or fn.__name__, self._ns)
+            fn_name = namespaced_id_ns(name or fn.__name__, self._namespaces)
 
             # fn is either a regular function or a RenderFunction object. If
             # it's the latter, we can give it a bit of metadata, which can be
diff --git a/tests/test_modules.py b/tests/test_modules.py
index 68167ac22..8067fe5c1 100644
--- a/tests/test_modules.py
+++ b/tests/test_modules.py
@@ -78,7 +78,6 @@ async def test_inputs_proxy():
         assert input_proxy.a() == 2
         assert input_proxy_proxy.a() == 3
         assert input_proxy_proxy["a"]() == 3
-        assert input["mod1_mod2_a"]() == 3
 
 
 def test_current_session():
@@ -129,12 +128,12 @@ def _():
     assert sessions["inner"] is sessions["inner_current"]
     assert sessions["inner_current"] is sessions["inner_calc_current"]
     assert isinstance(sessions["inner_current"], ModuleSession)
-    assert sessions["inner_current"]._ns == "mod_outer_mod_inner"
+    assert sessions["inner_current"]._namespaces == ["mod_outer", "mod_inner"]
 
     assert sessions["outer"] is sessions["outer_current"]
     assert sessions["outer_current"] is sessions["outer_calc_current"]
     assert isinstance(sessions["outer_current"], ModuleSession)
-    assert sessions["outer_current"]._ns == "mod_outer"
+    assert sessions["outer_current"]._namespaces == ["mod_outer"]
 
     assert sessions["top"] is sessions["top_current"]
     assert sessions["top_current"] is sessions["top_calc_current"]