4 files changed, 71 insertions, 2 deletions
diff --git a/doc/build/changelog/unreleased_20/9340.rst b/doc/build/changelog/unreleased_20/9340.rst
new file mode 100644
index 000000000..28cef6f64
--- /dev/null
+++ b/doc/build/changelog/unreleased_20/9340.rst
@@ -0,0 +1,6 @@
+.. change::
+    :tags: bug, typing
+    :tickets: 9340
+
+    Fixed typing issue where :func:`_orm.with_polymorphic` would not
+    record the class type correctly.
diff --git a/lib/sqlalchemy/orm/_orm_constructors.py b/lib/sqlalchemy/orm/_orm_constructors.py
index 3bd1db79d..64e7937f1 100644
--- a/lib/sqlalchemy/orm/_orm_constructors.py
+++ b/lib/sqlalchemy/orm/_orm_constructors.py
@@ -2208,7 +2208,7 @@ def aliased(
 
 
 def with_polymorphic(
-    base: Union[_O, Mapper[_O]],
+    base: Union[Type[_O], Mapper[_O]],
     classes: Union[Literal["*"], Iterable[Type[Any]]],
     selectable: Union[Literal[False, None], FromClause] = False,
     flat: bool = False,
diff --git a/lib/sqlalchemy/orm/util.py b/lib/sqlalchemy/orm/util.py
index ad9ce2013..1ef0d7159 100644
--- a/lib/sqlalchemy/orm/util.py
+++ b/lib/sqlalchemy/orm/util.py
@@ -994,7 +994,7 @@ class AliasedInsp(
     @classmethod
     def _with_polymorphic_factory(
         cls,
-        base: Union[_O, Mapper[_O]],
+        base: Union[Type[_O], Mapper[_O]],
         classes: Union[Literal["*"], Iterable[_EntityType[Any]]],
         selectable: Union[Literal[False, None], FromClause] = False,
         flat: bool = False,
diff --git a/test/ext/mypy/plain_files/issue_9340.py b/test/ext/mypy/plain_files/issue_9340.py
new file mode 100644
index 000000000..72dc72df1
--- /dev/null
+++ b/test/ext/mypy/plain_files/issue_9340.py
@@ -0,0 +1,63 @@
+from typing import Sequence
+from typing import TYPE_CHECKING
+
+from sqlalchemy import create_engine
+from sqlalchemy import select
+from sqlalchemy.orm import DeclarativeBase
+from sqlalchemy.orm import Mapped
+from sqlalchemy.orm import mapped_column
+from sqlalchemy.orm import Session
+from sqlalchemy.orm import with_polymorphic
+
+
+class Base(DeclarativeBase):
+    ...
+
+
+class Message(Base):
+    __tablename__ = "message"
+    __mapper_args__ = {
+        "polymorphic_on": "message_type",
+        "polymorphic_identity": __tablename__,
+    }
+    id: Mapped[int] = mapped_column(primary_key=True)
+    text: Mapped[str]
+    message_type: Mapped[str]
+
+
+class UserComment(Message):
+    __mapper_args__ = {
+        "polymorphic_identity": "user_comment",
+    }
+    username: Mapped[str] = mapped_column(nullable=True)
+
+
+engine = create_engine("postgresql+psycopg2://scott:tiger@localhost/")
+
+
+def get_messages() -> Sequence[Message]:
+    with Session(engine) as session:
+        message_query = select(Message)
+
+        if TYPE_CHECKING:
+            # EXPECTED_TYPE: Select[Tuple[Message]]
+            reveal_type(message_query)
+
+        return session.scalars(message_query).all()
+
+
+def get_poly_messages() -> Sequence[Message]:
+    with Session(engine) as session:
+        PolymorphicMessage = with_polymorphic(Message, (UserComment,))
+
+        if TYPE_CHECKING:
+            # EXPECTED_TYPE: AliasedClass[Message]
+            reveal_type(PolymorphicMessage)
+
+        poly_query = select(PolymorphicMessage)
+
+        if TYPE_CHECKING:
+            # EXPECTED_TYPE: Select[Tuple[Message]]
+            reveal_type(poly_query)
+
+        return session.scalars(poly_query).all()