diff --git a/src/rez/rex.py b/src/rez/rex.py
index da26ef518..1af391909 100644
--- a/src/rez/rex.py
+++ b/src/rez/rex.py
@@ -12,8 +12,8 @@
 from enum import Enum
 from contextlib import contextmanager
 from string import Formatter
-from collections.abc import MutableMapping
-from typing import Any, Iterable, Mapping
+from collections.abc import Mapping, MutableMapping
+from typing import Any, Iterable
 
 from rez.system import system
 from rez.config import config
@@ -200,7 +200,17 @@ def __init__(self, interpreter: ActionInterpreter,
         '''
         self.interpreter = interpreter
         self.verbose = verbose
-        self.parent_environ = os.environ if parent_environ is None else parent_environ
+        # On Windows env-var keys are case-insensitive; os.environ already
+        # behaves that way but a plain dict supplied by the caller does not,
+        # so wrap it on the Windows path only. See #2089.
+        self.parent_environ: Mapping[str, str]
+        if parent_environ is None:
+            self.parent_environ = os.environ
+        elif platform_.name == "windows" and not isinstance(
+                parent_environ, _CaseInsensitiveEnvironProxy):
+            self.parent_environ = _CaseInsensitiveEnvironProxy(parent_environ)
+        else:
+            self.parent_environ = parent_environ
         self.parent_variables = True if parent_variables is True \
             else set(parent_variables or [])
         self.environ = {}
@@ -447,6 +457,35 @@ def _keytoken(self, key):
         return self.interpreter.get_key_token(key)
 
 
+class _CaseInsensitiveEnvironProxy(Mapping):
+    """Read-only case-insensitive view over an env-var mapping.
+
+    Used by `ActionManager` to wrap a caller-supplied `parent_environ`
+    on Windows so rex lookups match native Windows env-var semantics
+    regardless of the casing used to populate the input dict.
+
+    Kept private to this module on purpose; promote to a shared util
+    only when a second caller needs it.
+    """
+    def __init__(self, data):
+        # keys are upper-cased once on the way in; same shape as
+        # os.environ on Windows. Last write wins on case collisions,
+        # which mirrors how os.environ resolves them too.
+        self._data = {k.upper(): v for k, v in data.items()}
+
+    def __getitem__(self, key):
+        return self._data[key.upper()]
+
+    def __contains__(self, key):
+        return isinstance(key, str) and key.upper() in self._data
+
+    def __iter__(self):
+        return iter(self._data)
+
+    def __len__(self):
+        return len(self._data)
+
+
 #===============================================================================
 # Interpreters
 #===============================================================================
diff --git a/src/rez/tests/test_rex.py b/src/rez/tests/test_rex.py
index ac3a8966d..8fe79c285 100644
--- a/src/rez/tests/test_rex.py
+++ b/src/rez/tests/test_rex.py
@@ -547,6 +547,39 @@ def test_intersects_ephemerals(self) -> None:
         self.assertRaises(RuntimeError,  # no default
                           intersects, ephemerals.get_range("foo.bar"), "0")
 
+    def test_parent_environ_case_insensitive_on_windows(self):
+        """Regression for #2089.
+
+        On Windows env vars are case-insensitive; os.environ already
+        behaves that way, but a plain dict copy of it doesn't. Passing
+        such a dict as parent_environ used to break env.<MixedCase>
+        lookups inside commands(). We now wrap on the Windows side.
+        """
+        from rez.utils.platform_ import platform_
+        if platform_.name != "windows":
+            self.skipTest("Windows-only behaviour")
+
+        env = {"SomeVariable": "covfefe"}
+        ex = self._create_executor(env)
+
+        # any casing of the key resolves
+        self.assertEqual(ex.manager.parent_environ["SomeVariable"], "covfefe")
+        self.assertEqual(ex.manager.parent_environ["SOMEVARIABLE"], "covfefe")
+        self.assertEqual(ex.manager.parent_environ["somevariable"], "covfefe")
+        self.assertIn("SOMEVARIABLE", ex.manager.parent_environ)
+        self.assertNotIn("OtherVariable", ex.manager.parent_environ)
+
+        # end-to-end: rex code references a different casing than the
+        # caller's dict and the lookup must succeed (pre-fix this
+        # raised RexUndefinedVariableError)
+        def _rex():
+            v = getenv("SOMEVARIABLE")  # noqa: F821 - rex builtin
+            setenv("RESULT", v)         # noqa: F821 - rex builtin
+
+        ex2 = self._create_executor({"SomeVariable": "covfefe"})
+        ex2.execute_function(_rex)
+        self.assertEqual(ex2.get_output().get("RESULT"), "covfefe")
+
 
 if __name__ == '__main__':
     unittest.main()