diff --git a/REMINDERS.md b/REMINDERS.md
index b904406..32db423 100644
--- a/REMINDERS.md
+++ b/REMINDERS.md
@@ -1,10 +1,17 @@
 # Reminders
 
+## v0.9.0
+
+- Complete deprecation of grammar rule `dtype_to_expand`.
+  Remove rule in `doctype.lark` and `DoctypeTranformer`.
+  Remove warning in `_app_generate_stubs::generate_stubs`.
+
+
 ## With Python >=3.13
 
-Remove vendored `glob_translate` in `docstub._vendored.stdlib`.
+- Remove vendored `glob_translate` in `docstub._vendored.stdlib`.
 
 
 ## With Python >=3.14
 
-Remove vendored `ProcessPoolExecutor` in `docstub._vendored.stdlib`.
+- Remove vendored `ProcessPoolExecutor` in `docstub._vendored.stdlib`.
diff --git a/src/docstub-stubs/doctype.pyi b/src/docstub-stubs/doctype.pyi
index e15111b..37c77d0 100644
--- a/src/docstub-stubs/doctype.pyi
+++ b/src/docstub-stubs/doctype.pyi
@@ -101,6 +101,7 @@ class DoctypeTransformer(lark.visitors.Transformer):
     def natlang_array(self, tree: lark.Tree) -> Expr: ...
     def array_name(self, tree: lark.Tree) -> Term: ...
     def dtype(self, tree: lark.Tree) -> Expr: ...
+    def dtype_union(self, tree: lark.Tree) -> Expr: ...
     def dtype_to_expand(self, tree: lark.Tree) -> Expr: ...
     def shape(self, tree: lark.Tree) -> Expr: ...
     def optional_info(self, tree: lark.Tree) -> lark.visitors._DiscardType: ...
diff --git a/src/docstub/_app_generate_stubs.py b/src/docstub/_app_generate_stubs.py
index 541b5b1..fb5fcf0 100644
--- a/src/docstub/_app_generate_stubs.py
+++ b/src/docstub/_app_generate_stubs.py
@@ -386,6 +386,18 @@ def generate_stubs(
 
     grammar = plugin_hooks.docstub_load_doctype_grammar()
     doctype_transformer = plugin_hooks.docstub_load_doctype_transformer()
+    if (doctype_transformer and hasattr(doctype_transformer, "dtype_to_expand")) or (
+        grammar and "dtype_to_expand" in grammar
+    ):
+        logger.warning(
+            "The grammar rule 'dtype_to_expand' is deprecated in favor of 'dtype_union'",
+            extra={
+                "details": "It will be removed in v0.9.0. Since `numpy.dtype` supports "
+                "unions, the rule was renamed. Use the rule 'dtype_union' if you want "
+                "to modify this expression in a plugin."
+            },
+        )
+
     doctype_transpiler = doctype.DoctypeTranspiler(
         grammar=grammar, transformer=doctype_transformer
     )
diff --git a/src/docstub/doctype.lark b/src/docstub/doctype.lark
index 65015ac..30d8962 100644
--- a/src/docstub/doctype.lark
+++ b/src/docstub/doctype.lark
@@ -126,11 +126,15 @@ ARRAY_NAME: "array" | "ndarray" | "array-like" | "array_like"
 
 
 // The dtype used in an array expression.
-dtype: qualname | dtype_to_expand
+dtype: qualname | dtype_union | dtype_to_expand
 
 
-// Dtypes may be specified as a union. But I'm not sure if `np.dtype` accepts
-// a union. So we use expansion here instead.
+// Dtypes may be specified as a union. But we don't want to allow any `_type`,
+// so we define a dtype-specific union.
+dtype_union.1: "(" qualname (_OR qualname)+ ")"
+
+
+// DEPRECATED in favor of `dtype_union`, will be removed in v0.9.0.
 dtype_to_expand: "(" qualname (_OR qualname)+ ")"
 
 
diff --git a/src/docstub/doctype.py b/src/docstub/doctype.py
index 610c07a..bd306f7 100644
--- a/src/docstub/doctype.py
+++ b/src/docstub/doctype.py
@@ -709,6 +709,18 @@ def dtype(self, tree):
         expr = self._create_subscription_expr(children, rule="dtype")
         return expr
 
+    def dtype_union(self, tree):
+        """
+        Parameters
+        ----------
+        tree : lark.Tree
+
+        Returns
+        -------
+        out : Expr
+        """
+        return self.union(tree)
+
     def dtype_to_expand(self, tree):
         """
         Parameters
diff --git a/tests/test_doctype.py b/tests/test_doctype.py
index dd794da..f457a66 100644
--- a/tests/test_doctype.py
+++ b/tests/test_doctype.py
@@ -357,20 +357,20 @@ def test_malformed_shape(self, shape):
             ("ndarray of dtype int", "ndarray[tuple[Any, ...], np.dtype[int]]"),
             (
                 "ndarray of dtype (int or float)",
-                "ndarray[tuple[Any, ...], np.dtype[int]]"
-                " | ndarray[tuple[Any, ...], np.dtype[float]]",
+                "ndarray[tuple[Any, ...], np.dtype[int | float]]",
             ),
             (
                 "ndarray of dtype (int | float | complex)",
-                "ndarray[tuple[Any, ...], np.dtype[int]]"
-                " | ndarray[tuple[Any, ...], np.dtype[float]]"
-                " | ndarray[tuple[Any, ...], np.dtype[complex]]",
+                "ndarray[tuple[Any, ...], np.dtype[int | float | complex]]",
             ),
         ],
     )
     def test_dtype(self, doctype, expected):
         expr = DoctypeTranspiler()(doctype)
         assert expr.as_code() == expected
+        assert expr.find_one(rule="dtype")
+        if " or " in doctype:
+            assert expr.find_one(rule="union")
 
     @pytest.mark.parametrize(
         ("doctype", "expected"),
@@ -381,10 +381,8 @@ def test_dtype(self, doctype, expected):
             ),
             (
                 "ndarray of dtype (int or float) and shape ([N,] M)",
-                "ndarray[tuple[int], np.dtype[int]]"
-                " | ndarray[tuple[int], np.dtype[float]]"
-                " | ndarray[tuple[int, int], np.dtype[int]]"
-                " | ndarray[tuple[int, int], np.dtype[float]]",
+                "ndarray[tuple[int], np.dtype[int | float]]"
+                " | ndarray[tuple[int, int], np.dtype[int | float]]",
             ),
         ],
     )