diff --git a/news/7633.bugfix.md b/news/7633.bugfix.md
new file mode 100644
index 00000000000..28d97bd6f95
--- /dev/null
+++ b/news/7633.bugfix.md
@@ -0,0 +1 @@
+Raise a `VarAttributeError` compile-time error when accessing an unserialized property (not decorated with `@pydantic.computed_field`) on a Pydantic model or dataclass within a state var, including when the state var type is a Union.
\ No newline at end of file
diff --git a/packages/reflex-base/src/reflex_base/utils/types.py b/packages/reflex-base/src/reflex_base/utils/types.py
index eead8d73f30..7c60f994896 100644
--- a/packages/reflex-base/src/reflex_base/utils/types.py
+++ b/packages/reflex-base/src/reflex_base/utils/types.py
@@ -972,6 +972,78 @@ def get_attribute_descriptor(cls: GenericType, name: str) -> Any:
         return None
 
 
+def is_unserialized_property(
+    cls: GenericType,
+    name: str,
+    descriptor: Any = _NO_DESCRIPTOR,
+) -> bool:
+    """Check whether a property is omitted by the default object serializer.
+
+    Args:
+        cls: The type being inspected.
+        name: The attribute name.
+        descriptor: A previously resolved descriptor, if available.
+
+    Returns:
+        Whether the property is omitted by the default serializer.
+    """
+    if is_union(cls):
+        return any(
+            is_unserialized_property(arg, name)
+            for arg in get_args(cls)
+            if arg is not type(None)
+        )
+
+    cls = get_origin(cls) or cls
+    if not isinstance(cls, type):
+        return False
+
+    attr = (
+        get_attribute_descriptor(cls, name)
+        if descriptor is _NO_DESCRIPTOR
+        else descriptor
+    )
+
+    if not isinstance(attr, property):
+        return False
+
+    from reflex_base.utils import serializers
+
+    serializer = serializers.get_serializer(cls)
+
+    if dataclasses.is_dataclass(cls):
+        if serializer is not None:
+            return False
+
+        return name not in {field.name for field in dataclasses.fields(cls)}
+
+    model_fields = getattr(cls, "model_fields", None)
+    if not isinstance(model_fields, dict):
+        model_fields = getattr(cls, "__fields__", None)
+
+        if not isinstance(model_fields, dict):
+            return False
+
+    pydantic_decorators = getattr(cls, "__pydantic_decorators__", None)
+    if getattr(pydantic_decorators, "model_serializers", None):
+        return False
+
+    default_serializers = tuple(
+        default
+        for default in (
+            getattr(serializers, "serialize_base_model", None),
+            serializers.serialize_sqlmodel,
+        )
+        if default is not None
+    )
+    if serializer not in default_serializers:
+        return False
+
+    computed_fields = getattr(cls, "model_computed_fields", {}) or {}
+
+    return name not in model_fields and name not in computed_fields
+
+
 def get_attribute_access_type(
     cls: GenericType, name: str, descriptor: Any = _NO_DESCRIPTOR
 ) -> GenericType | None:
diff --git a/packages/reflex-base/src/reflex_base/vars/object.py b/packages/reflex-base/src/reflex_base/vars/object.py
index 8b8504944df..d588384d407 100644
--- a/packages/reflex-base/src/reflex_base/vars/object.py
+++ b/packages/reflex-base/src/reflex_base/vars/object.py
@@ -329,22 +329,75 @@ def __getattr__(self, name: str) -> Var:
         var_type = types.value_inside_optional(var_type)
 
         fixed_type = get_origin(var_type) or var_type
+        descriptor = None
+        is_union_type = types.is_union(var_type)
 
-        if isinstance(fixed_type, type) and not safe_issubclass(fixed_type, Mapping):
-            # Resolve the raw descriptor once and reuse it. A HybridProperty resolves to a
-            # frontend Var with this object var substituted as `self` (e.g. `State.info.a_b`);
-            # any other descriptor is passed to `get_attribute_access_type` so the class
-            # lookup is not repeated.
-            descriptor = types.get_attribute_descriptor(fixed_type, name)
-            if isinstance(descriptor, HybridProperty):
-                hybrid_var = descriptor._get_var(self)
-                if hybrid_var is None:
+        if (isinstance(fixed_type, type) or is_union_type) and not safe_issubclass(
+            fixed_type, Mapping
+        ):
+            if not is_union_type:
+                # Resolve the raw descriptor once and reuse it. A HybridProperty resolves to a
+                # frontend Var with this object var substituted as `self` (e.g. `State.info.a_b`);
+                # any other descriptor is passed to `get_attribute_access_type` so the class
+                # lookup is not repeated.
+                descriptor = types.get_attribute_descriptor(fixed_type, name)
+                if isinstance(descriptor, HybridProperty):
+                    hybrid_var = descriptor._get_var(self)
+                    if hybrid_var is None:
+                        msg = (
+                            f"The hybrid property '{name}' of {fixed_type.__name__} has no "
+                            f"frontend value, so it cannot be accessed on `{self!s}`."
+                        )
+                        raise VarAttributeError(msg)
+                    return hybrid_var
+
+            if types.is_unserialized_property(var_type, name):
+                property_types = get_args(var_type) if is_union_type else (fixed_type,)
+
+                property_type = next(
+                    (
+                        t
+                        for t in property_types
+                        if isinstance(t, type)
+                        and isinstance(
+                            types.get_attribute_descriptor(t, name), property
+                        )
+                        and types.is_unserialized_property(t, name)
+                    ),
+                    fixed_type,
+                )
+
+                if is_union_type:
+                    msg = (
+                        f"The property '{name}' on union type "
+                        f"'{var_type}' may not be included in serialized state."
+                    )
+                elif isinstance(property_type, type) and dataclasses.is_dataclass(
+                    property_type
+                ):
+                    msg = (
+                        f"The property '{name}' on dataclass "
+                        f"'{property_type.__name__}' is not included in "
+                        "serialized state. Only declared dataclass fields "
+                        "are serialized."
+                    )
+                elif isinstance(property_type, type):
                     msg = (
-                        f"The hybrid property '{name}' of {fixed_type.__name__} has no "
-                        f"frontend value, so it cannot be accessed on `{self!s}`."
+                        f"The property '{name}' on Pydantic model "
+                        f"'{property_type.__name__}' is not included in "
+                        "serialized state. Use "
+                        "`@pydantic.computed_field` to include the "
+                        "computed value in model_dump()."
                     )
-                    raise VarAttributeError(msg)
-                return hybrid_var
+                else:
+                    msg = (
+                        f"The property '{name}' on type '{var_type}' "
+                        "may not be included in serialized state."
+                    )
+
+                raise VarAttributeError(msg)
+
+        if isinstance(fixed_type, type) and not safe_issubclass(fixed_type, Mapping):
             attribute_type = get_attribute_access_type(var_type, name, descriptor)
         elif is_typeddict(fixed_type) or fixed_type in types.UnionTypes:
             attribute_type = get_attribute_access_type(var_type, name)
@@ -353,8 +406,8 @@ def __getattr__(self, name: str) -> Var:
 
         if attribute_type is None:
             msg = (
-                f"The State var `{self!s}` of type {self._var_type} has no attribute '{name}' or may have been annotated "
-                f"wrongly."
+                f"The State var `{self!s}` of type {self._var_type} has no attribute "
+                f"'{name}' or may have been annotated wrongly."
             )
             raise VarAttributeError(msg)
         return ObjectItemOperation.create(self, name, attribute_type).guess_type()
diff --git a/tests/units/vars/test_object.py b/tests/units/vars/test_object.py
index 24d384d1f85..809cd3ee0e8 100644
--- a/tests/units/vars/test_object.py
+++ b/tests/units/vars/test_object.py
@@ -1,6 +1,6 @@
 import dataclasses
 from collections.abc import Sequence
-from typing import Annotated, Any, Literal, assert_type
+from typing import Annotated, Any, Literal, Union, assert_type
 
 import pytest
 from reflex_base.utils.exceptions import VarAttributeError
@@ -378,3 +378,139 @@ def test_annotated_var_type_is_unwrapped() -> None:
     assert isinstance(var, ObjectVar)
     assert var._var_type is Base
     assert var.quantity._var_type is int
+
+
+def test_plain_property_on_pydantic_model_raises() -> None:
+    """Plain Pydantic properties are not included in serialized state."""
+
+    class Item(pydantic.BaseModel):
+        name: str = "solo"
+
+        @property
+        def label(self) -> str:
+            return f"{self.name}!"
+
+        @pydantic.computed_field
+        @property
+        def tag(self) -> str:
+            return f"#{self.name}"
+
+    class ItemState(rx.State):
+        item: rx.Field[Item] = rx.field(Item())
+
+    with pytest.raises(VarAttributeError, match="computed_field"):
+        _ = ItemState.item.label
+
+    assert ItemState.item.name._var_type is str
+    assert ItemState.item.tag._var_type is str
+
+
+def test_plain_property_on_dataclass_raises() -> None:
+    """Plain dataclass properties are not included in serialized state."""
+
+    @dataclasses.dataclass
+    class Point:
+        x: int = 1
+
+        @property
+        def double(self) -> int:
+            return self.x * 2
+
+    class PointState(rx.State):
+        point: rx.Field[Point] = rx.field(Point())
+
+    with pytest.raises(
+        VarAttributeError,
+        match="declared dataclass fields",
+    ):
+        _ = PointState.point.double
+
+    assert PointState.point.x._var_type is int
+
+
+def test_plain_property_on_pydantic_model_with_custom_serializer() -> None:
+    """Plain properties remain accessible when a custom serializer can include them."""
+
+    class Item(pydantic.BaseModel):
+        name: str = "solo"
+
+        @property
+        def label(self) -> str:
+            return f"{self.name}!"
+
+        @pydantic.model_serializer
+        def serialize_model(self) -> dict[str, str]:
+            return {
+                "name": self.name,
+                "label": self.label,
+            }
+
+    class ItemState(rx.State):
+        item: rx.Field[Item] = rx.field(Item())
+
+    assert Item().model_dump() == {"name": "solo", "label": "solo!"}
+    assert ItemState.item.label._var_type is str
+
+
+def test_plain_property_on_union_of_pydantic_models_raises() -> None:
+    """Plain properties are rejected when either union branch omits them."""
+
+    class ItemA(pydantic.BaseModel):
+        name: str = "a"
+
+        @property
+        def label(self) -> str:
+            return f"{self.name}!"
+
+    class ItemB(pydantic.BaseModel):
+        name: str = "b"
+
+        @property
+        def label(self) -> str:
+            return f"{self.name}?"
+
+    class UnionState(rx.State):
+        item: rx.Field[ItemA | ItemB] = rx.field(ItemA())
+
+    with pytest.raises(VarAttributeError, match="union type"):
+        _ = UnionState.item.label
+
+
+def test_unserialized_property_checks_union_branches() -> None:
+    """The property guard detects an unserialized property in either branch."""
+
+    from reflex_base.utils import types
+
+    class ItemA(pydantic.BaseModel):
+        @property
+        def label(self) -> str:
+            return "a"
+
+    class ItemB(pydantic.BaseModel):
+        @property
+        def label(self) -> str:
+            return "b"
+
+    assert types.is_unserialized_property(ItemA, "label")
+    assert types.is_unserialized_property(ItemB, "label")
+    assert types.is_unserialized_property(ItemA | ItemB, "label")
+
+
+def test_plain_property_on_typing_union_raises() -> None:
+    """Plain properties are rejected on typing.Union model types."""
+
+    class ItemA(pydantic.BaseModel):
+        @property
+        def label(self) -> str:
+            return "a"
+
+    class ItemB(pydantic.BaseModel):
+        @property
+        def label(self) -> str:
+            return "b"
+
+    class UnionState(rx.State):
+        item: rx.Field[Union[ItemA, ItemB]] = rx.field(ItemA())  # noqa: UP007
+
+    with pytest.raises(VarAttributeError, match="union type"):
+        _ = UnionState.item.label