diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index 80f9c76e90..ed5959f0cf 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -745,10 +745,9 @@ def _get_multidict_value(
             and len(value) == 0
         )
     ):
-        if field.field_info.is_required():
-            return
-        else:
-            return deepcopy(field.default)
+        # Don't pre-fill defaults here - let Pydantic handle them during validation
+        # This preserves model_fields_set correctly for Form fields
+        return None
     return value
 
 
diff --git a/tests/test_form_model_fields_set.py b/tests/test_form_model_fields_set.py
new file mode 100644
index 0000000000..9bae51e022
--- /dev/null
+++ b/tests/test_form_model_fields_set.py
@@ -0,0 +1,120 @@
+"""
+Test that Form-based dependency models preserve model_fields_set correctly.
+
+This test addresses issue https://github.com/fastapi/fastapi/issues/13399
+where Form models were pre-filling default values, making it impossible
+to distinguish between explicitly set fields and fields using defaults.
+"""
+
+from typing import Annotated
+
+import pytest
+from fastapi import FastAPI, Form
+from fastapi.testclient import TestClient
+from pydantic import BaseModel
+
+
+class ModelWithDefaults(BaseModel):
+    field_bool: bool = True
+    field_str: str = "default_value"
+    field_int: int = 42
+
+
+app = FastAPI()
+
+
+@app.post("/json")
+async def json_endpoint(model: ModelWithDefaults):
+    return {
+        "fields_set": list(model.model_fields_set),
+        "field_bool": model.field_bool,
+        "field_str": model.field_str,
+        "field_int": model.field_int,
+    }
+
+
+@app.post("/form")
+async def form_endpoint(model: Annotated[ModelWithDefaults, Form()]):
+    return {
+        "fields_set": list(model.model_fields_set),
+        "field_bool": model.field_bool,
+        "field_str": model.field_str,
+        "field_int": model.field_int,
+    }
+
+
+client = TestClient(app)
+
+
+def test_json_empty_preserves_fields_set():
+    """JSON with empty dict should have empty fields_set"""
+    response = client.post("/json", json={})
+    assert response.status_code == 200
+    data = response.json()
+    assert data["fields_set"] == []
+    assert data["field_bool"] is True
+    assert data["field_str"] == "default_value"
+    assert data["field_int"] == 42
+
+
+def test_json_partial_preserves_fields_set():
+    """JSON with partial data should only set provided fields"""
+    response = client.post("/json", json={"field_str": "custom"})
+    assert response.status_code == 200
+    data = response.json()
+    assert data["fields_set"] == ["field_str"]
+    assert data["field_bool"] is True
+    assert data["field_str"] == "custom"
+    assert data["field_int"] == 42
+
+
+def test_json_all_fields_preserves_fields_set():
+    """JSON with all fields should mark all as set"""
+    response = client.post(
+        "/json",
+        json={"field_bool": False, "field_str": "custom", "field_int": 100},
+    )
+    assert response.status_code == 200
+    data = response.json()
+    assert set(data["fields_set"]) == {"field_bool", "field_str", "field_int"}
+    assert data["field_bool"] is False
+    assert data["field_str"] == "custom"
+    assert data["field_int"] == 100
+
+
+def test_form_empty_preserves_fields_set():
+    """Form with empty data should have empty fields_set (matching JSON behavior)"""
+    response = client.post("/form", data={})
+    assert response.status_code == 200
+    data = response.json()
+    # This is the key fix: Form should behave like JSON
+    assert data["fields_set"] == []
+    assert data["field_bool"] is True
+    assert data["field_str"] == "default_value"
+    assert data["field_int"] == 42
+
+
+def test_form_partial_preserves_fields_set():
+    """Form with partial data should only set provided fields (matching JSON behavior)"""
+    response = client.post("/form", data={"field_str": "custom"})
+    assert response.status_code == 200
+    data = response.json()
+    # This is the key fix: only field_str should be in fields_set
+    assert data["fields_set"] == ["field_str"]
+    assert data["field_bool"] is True
+    assert data["field_str"] == "custom"
+    assert data["field_int"] == 42
+
+
+def test_form_all_fields_preserves_fields_set():
+    """Form with all fields should mark all as set"""
+    response = client.post(
+        "/form",
+        data={"field_bool": "false", "field_str": "custom", "field_int": "100"},
+    )
+    assert response.status_code == 200
+    data = response.json()
+    assert set(data["fields_set"]) == {"field_bool", "field_str", "field_int"}
+    assert data["field_bool"] is False
+    assert data["field_str"] == "custom"
+    assert data["field_int"] == 100