diff --git a/src/marshmallow_dataframe/split.py b/src/marshmallow_dataframe/split.py
index b988e67..9fc4db4 100644
--- a/src/marshmallow_dataframe/split.py
+++ b/src/marshmallow_dataframe/split.py
@@ -31,7 +31,11 @@ class SplitDataFrameSchemaMeta(DataFrameSchemaMeta):
                 else dtype_to_field(index_dtype)
             )
 
-            fields["index"] = ma.fields.List(index_field, required=True)
+            index_required = False if index_dtype is None else True
+
+            fields["index"] = ma.fields.List(
+                index_field, required=index_required
+            )
 
             fields["columns"] = ma.fields.List(
                 ma.fields.String,
@@ -51,7 +55,9 @@ class SplitDataFrameSchema(ma.Schema, metaclass=SplitDataFrameSchemaMeta):
 
     @ma.validates_schema(skip_on_field_errors=True)
     def validate_index_data_length(self, data: dict, **kwargs) -> None:
-        if len(data["index"]) != len(data["data"]):
+        if data.get("index") is not None and len(data["index"]) != len(
+            data["data"]
+        ):
             raise ma.ValidationError(
                 "Length of `index` and `data` must be equal.", "data"
             )

Should index be mandatory on split format?
Hi, and thanks for the work on this package ! 
I am also trying to get that working nicely together...

I recently encountered : 
```
{'index': ['Missing data for required field.']}
```
But I expected index to be optional (when not present assume range(n), same as pandas.DataFrame()

What do you think ?

diff --git a/tests/test_split.py b/tests/test_split.py
index ba2154f..ac975d4 100644
--- a/tests/test_split.py
+++ b/tests/test_split.py
@@ -114,6 +114,23 @@ def test_split_schema_missing_top_level_key(
     assert exc.value.messages[key] == ["Missing data for required field."]
 
 
+@pytest.mark.parametrize("require_index", [True, False])
+def test_optional_index_field(sample_df, require_index):
+    class MySchema(SplitDataFrameSchema):
+        class Meta:
+            dtypes = sample_df.dtypes
+            index_dtype = sample_df.index.dtype if require_index else None
+
+    serialized_df = serialize_df(sample_df, orient="split")
+
+    if not require_index:
+        del serialized_df["index"]
+
+    output = MySchema().load(serialized_df)
+
+    assert_frame_equal(output, sample_df)
+
+
 def test_split_schema_str_index(sample_df):
     test_df = sample_df.copy()
     test_df.index = test_df.index.astype(str)

pip install -e .
parse_log_pytest
requirements.txt
pytest hypothesis[numpy]
3.7
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs==24.2.0
certifi @ file:///croot/certifi_1671487769961/work/certifi
exceptiongroup==1.2.2
hypothesis==6.79.4
importlib-metadata==6.7.0
iniconfig==2.0.0
marshmallow==3.19.0
-e git+https://github.com/facultyai/marshmallow-dataframe.git@36253b6b2006839f83621c7eee83d0a544a3dce8#egg=marshmallow_dataframe
numpy==1.21.6
packaging==24.0
pandas==1.3.5
pluggy==1.2.0
pytest==7.4.4
python-dateutil==2.9.0.post0
pytz==2025.2
six==1.17.0
sortedcontainers==2.4.0
tomli==2.0.1
typing_extensions==4.7.1
zipp==3.15.0

name: marshmallow-dataframe
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2022.12.7=py37h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - pip=22.3.1=py37h06a4308_0
  - python=3.7.16=h7a1cb2a_0
  - readline=8.2=h5eee18b_0
  - setuptools=65.6.3=py37h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.38.4=py37h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - attrs==24.2.0
      - exceptiongroup==1.2.2
      - hypothesis==6.79.4
      - importlib-metadata==6.7.0
      - iniconfig==2.0.0
      - marshmallow==3.19.0
      - marshmallow-dataframe==0.1.3
      - numpy==1.21.6
      - packaging==24.0
      - pandas==1.3.5
      - pluggy==1.2.0
      - pytest==7.4.4
      - python-dateutil==2.9.0.post0
      - pytz==2025.2
      - six==1.17.0
      - sortedcontainers==2.4.0
      - tomli==2.0.1
      - typing-extensions==4.7.1
      - zipp==3.15.0
prefix: /opt/conda/envs/marshmallow-dataframe

tests/test_split.py::test_optional_index_field[False]
tests/test_split.py::test_schema_no_dtypes tests/test_split.py::test_schema_wrong_dtypes tests/test_split.py::test_split_schema tests/test_split.py::test_split_schema_hypothesis tests/test_split.py::test_split_schema_missing_top_level_key[data] tests/test_split.py::test_split_schema_missing_top_level_key[index] tests/test_split.py::test_split_schema_missing_top_level_key[columns] tests/test_split.py::test_optional_index_field[True] tests/test_split.py::test_split_schema_str_index tests/test_split.py::test_split_schema_missing_column tests/test_split.py::test_split_schema_swapped_column tests/test_split.py::test_split_schema_wrong_row_length tests/test_split.py::test_split_schema_wrong_type_in_data tests/test_split.py::test_split_schema_index_data_length_mismatch[index] tests/test_split.py::test_split_schema_index_data_length_mismatch[data] tests/test_split.py::test_split_schema_wrong_category[categorical_int-data0] tests/test_split.py::test_split_schema_wrong_category[categorical_str-data1]
src/marshmallow_dataframe/split.py
src/marshmallow_dataframe/split.py