diff --git a/flytekit/core/type_engine.py b/flytekit/core/type_engine.py
index 900afa8562..897c5a6400 100644
--- a/flytekit/core/type_engine.py
+++ b/flytekit/core/type_engine.py
@@ -676,12 +676,33 @@ def _make_dataclass_serializable(self, python_val: T, python_type: Type[T]) -> t
         """
         from flytekit.types.directory import FlyteDirectory
         from flytekit.types.file import FlyteFile
+        from flytekit.types.structured import StructuredDataset
 
         # Handle Optional
         if UnionTransformer.is_optional_type(python_type):
-            if python_val is None:
-                return None
-            return self._make_dataclass_serializable(python_val, get_args(python_type)[0])
+
+            def get_expected_type(python_val: T, types: tuple) -> Type[T | None]:
+                if len(set(types) & {FlyteFile, FlyteDirectory, StructuredDataset}) > 1:
+                    raise ValueError(
+                        "Cannot have more than one Flyte type in the Union when attempting to use the string shortcut. Please specify the full object (e.g. FlyteFile(...)) instead of just passing a string."
+                    )
+
+                for t in types:
+                    try:
+                        trans = TypeEngine.get_transformer(t)  # type: ignore
+                        if trans:
+                            trans.assert_type(t, python_val)
+                            return t
+                    except Exception:
+                        continue
+                return type(None)
+
+            # Get the expected type in the Union type
+            expected_type = type(None)
+            if python_val is not None:
+                expected_type = get_expected_type(python_val, get_args(python_type))  # type: ignore
+
+            return self._make_dataclass_serializable(python_val, expected_type)
 
         if hasattr(python_type, "__origin__") and get_origin(python_type) is list:
             if python_val is None:
diff --git a/tests/flytekit/unit/core/test_dataclass.py b/tests/flytekit/unit/core/test_dataclass.py
index 58dfcd1e45..4e098c254b 100644
--- a/tests/flytekit/unit/core/test_dataclass.py
+++ b/tests/flytekit/unit/core/test_dataclass.py
@@ -1118,3 +1118,17 @@ def empty_nested_dc_wf() -> NestedFlyteTypes:
 
     empty_nested_flyte_types = empty_nested_dc_wf()
     DataclassTransformer().assert_type(NestedFlyteTypes, empty_nested_flyte_types)
+
+def test_dataclass_serialize_with_multiple_dataclass_union():
+    @dataclass
+    class A():
+        x: int
+
+    @dataclass
+    class B():
+        x: FlyteFile
+
+    b = B(x="s3://my-bucket/my-file")
+    res = DataclassTransformer()._make_dataclass_serializable(b, Union[None, A, B])
+
+    assert res.x.path == "s3://my-bucket/my-file"
diff --git a/tests/flytekit/unit/core/test_flytetypes.py b/tests/flytekit/unit/core/test_flytetypes.py
new file mode 100644
index 0000000000..366c3547c7
--- /dev/null
+++ b/tests/flytekit/unit/core/test_flytetypes.py
@@ -0,0 +1,17 @@
+from dataclasses import dataclass
+from flytekit.types.file import FlyteFile
+from flytekit.types.structured.structured_dataset import StructuredDataset
+from flytekit.core.type_engine import DataclassTransformer
+from typing import Union
+import pytest
+import re
+
+def test_dataclass_union_with_multiple_flytetypes_error():
+    @dataclass
+    class DC():
+        x: Union[None, StructuredDataset, FlyteFile]
+
+
+    dc = DC(x="s3://my-bucket/my-file")
+    with pytest.raises(ValueError, match=re.escape("Cannot have more than one Flyte type in the Union when attempting to use the string shortcut. Please specify the full object (e.g. FlyteFile(...)) instead of just passing a string.")):
+        DataclassTransformer()._make_dataclass_serializable(dc, DC)
diff --git a/tests/flytekit/unit/core/test_type_engine.py b/tests/flytekit/unit/core/test_type_engine.py
index 8721a8d4db..cb641eebe4 100644
--- a/tests/flytekit/unit/core/test_type_engine.py
+++ b/tests/flytekit/unit/core/test_type_engine.py
@@ -967,6 +967,7 @@ class TestFileStruct(DataClassJsonMixin):
         b: typing.Optional[FlyteFile]
         b_prime: typing.Optional[FlyteFile]
         c: typing.Union[FlyteFile, None]
+        c_prime: typing.Union[None, FlyteFile]
         d: typing.List[FlyteFile]
         e: typing.List[typing.Optional[FlyteFile]]
         e_prime: typing.List[typing.Optional[FlyteFile]]
@@ -989,6 +990,7 @@ class TestFileStruct(DataClassJsonMixin):
             b=f1,
             b_prime=None,
             c=f1,
+            c_prime=f1,
             d=[f1],
             e=[f1],
             e_prime=[None],
@@ -1011,6 +1013,7 @@ class TestFileStruct(DataClassJsonMixin):
         assert dict_obj["b"]["path"] == remote_path
         assert dict_obj["b_prime"] is None
         assert dict_obj["c"]["path"] == remote_path
+        assert dict_obj["c_prime"]["path"] == remote_path
         assert dict_obj["d"][0]["path"] == remote_path
         assert dict_obj["e"][0]["path"] == remote_path
         assert dict_obj["e_prime"][0] is None
@@ -1028,6 +1031,7 @@ class TestFileStruct(DataClassJsonMixin):
         assert o.b.remote_path == ot.b.remote_source
         assert ot.b_prime is None
         assert o.c.remote_path == ot.c.remote_source
+        assert o.c_prime.remote_path == ot.c_prime.remote_source
         assert o.d[0].remote_path == ot.d[0].remote_source
         assert o.e[0].remote_path == ot.e[0].remote_source
         assert o.e_prime == [None]