diff --git a/metaflow/plugins/cards/card_cli.py b/metaflow/plugins/cards/card_cli.py
index 9cb8b4bbb9d..70ca05d0a67 100644
--- a/metaflow/plugins/cards/card_cli.py
+++ b/metaflow/plugins/cards/card_cli.py
@@ -15,6 +15,7 @@
 import json
 import uuid
 import signal
+import sys
 import random
 from contextlib import contextmanager
 from functools import wraps
@@ -161,8 +162,11 @@ def resolve_card(
 @contextmanager
 def timeout(time):
     # Register a function to raise a TimeoutError on the signal.
+    if sys.platform == "win32":
+        raise RuntimeError(
+            "Card timeout is not supported on Windows (SIGALRM is POSIX-only)."
+        )
     signal.signal(signal.SIGALRM, raise_timeout)
-    # Schedule the signal to be sent after ``time``.
     signal.alarm(time)
 
     try:
diff --git a/metaflow/plugins/timeout_decorator.py b/metaflow/plugins/timeout_decorator.py
index 91c1814ff33..0855db9830a 100644
--- a/metaflow/plugins/timeout_decorator.py
+++ b/metaflow/plugins/timeout_decorator.py
@@ -1,4 +1,5 @@
 import signal
+import sys
 import traceback
 
 from metaflow.exception import MetaflowException
@@ -70,6 +71,12 @@ def task_pre_step(
         if ubf_context != UBF_CONTROL and retry_count <= max_user_code_retries:
             # enable timeout only when executing user code
             self.step_name = step_name
+            if sys.platform == "win32":
+                raise MetaflowException(
+                    "@timeout is not supported on Windows because it relies on "
+                    "POSIX signals (SIGALRM). Use a Linux-based compute backend "
+                    "(e.g. @kubernetes or @batch) to use @timeout."
+                )
             signal.signal(signal.SIGALRM, self._sigalrm_handler)
             signal.alarm(self.secs)