diff --git a/aws_lambda_powertools/utilities/parser/functions.py b/aws_lambda_powertools/utilities/parser/functions.py
index b9a35176a1e..351e214da93 100644
--- a/aws_lambda_powertools/utilities/parser/functions.py
+++ b/aws_lambda_powertools/utilities/parser/functions.py
@@ -35,12 +35,17 @@ def _retrieve_or_set_model_from_cache(model: type[T]) -> TypeAdapter:
         The TypeAdapter instance for the given model,
         either retrieved from the cache or newly created and stored in the cache.
     """
+
     id_model = id(model)
 
     if id_model in CACHE_TYPE_ADAPTER:
         return CACHE_TYPE_ADAPTER[id_model]
 
-    CACHE_TYPE_ADAPTER[id_model] = TypeAdapter(model)
+    if isinstance(model, TypeAdapter):
+        CACHE_TYPE_ADAPTER[id_model] = model
+    else:
+        CACHE_TYPE_ADAPTER[id_model] = TypeAdapter(model)
+
     return CACHE_TYPE_ADAPTER[id_model]
 
 
diff --git a/tests/functional/parser/test_parser.py b/tests/functional/parser/test_parser.py
index c7c90b70265..a6857af520d 100644
--- a/tests/functional/parser/test_parser.py
+++ b/tests/functional/parser/test_parser.py
@@ -154,7 +154,41 @@ class FailedCallback(pydantic.BaseModel):
     DogCallback = Annotated[Union[SuccessfulCallback, FailedCallback], pydantic.Field(discriminator="status")]
 
     @event_parser(model=DogCallback)
-    def handler(event: test_input, _: Any) -> str:
+    def handler(event, _: Any) -> str:
+        if isinstance(event, FailedCallback):
+            return f"Uh oh. Had a problem: {event.error}"
+
+        return f"Successfully retrieved {event.breed} named {event.name}"
+
+    ret = handler(test_input, None)
+    assert ret == expected
+
+
+@pytest.mark.parametrize(
+    "test_input,expected",
+    [
+        (
+            {"status": "succeeded", "name": "Clifford", "breed": "Labrador"},
+            "Successfully retrieved Labrador named Clifford",
+        ),
+        ({"status": "failed", "error": "oh some error"}, "Uh oh. Had a problem: oh some error"),
+    ],
+)
+def test_parser_unions_with_type_adapter_instance(test_input, expected):
+    class SuccessfulCallback(pydantic.BaseModel):
+        status: Literal["succeeded"]
+        name: str
+        breed: Literal["Newfoundland", "Labrador"]
+
+    class FailedCallback(pydantic.BaseModel):
+        status: Literal["failed"]
+        error: str
+
+    DogCallback = Annotated[Union[SuccessfulCallback, FailedCallback], pydantic.Field(discriminator="status")]
+    DogCallbackTypeAdapter = pydantic.TypeAdapter(DogCallback)
+
+    @event_parser(model=DogCallbackTypeAdapter)
+    def handler(event, _: Any) -> str:
         if isinstance(event, FailedCallback):
             return f"Uh oh. Had a problem: {event.error}"