diff --git a/onnxmltools/convert/lightgbm/_parse.py b/onnxmltools/convert/lightgbm/_parse.py
index f4c21eef4..83ad6e455 100644
--- a/onnxmltools/convert/lightgbm/_parse.py
+++ b/onnxmltools/convert/lightgbm/_parse.py
@@ -102,7 +102,7 @@ def _get_lightgbm_operator_name(model):
     return lightgbm_operator_name_map[model_type]
 
 
-def _parse_lightgbm_simple_model(scope, model, inputs, split=None):
+def _parse_lightgbm_simple_model(scope, model, inputs, split=None, decision_leaf=False):
     """
     This function handles all non-pipeline models.
 
@@ -111,11 +111,14 @@ def _parse_lightgbm_simple_model(scope, model, inputs, split=None):
     :param inputs: A list of variables
     :param split: split TreeEnsembleRegressor into multiple node to reduce
         discrepancies
+    :param decision_leaf: if True, an additional output is added to return
+        the leaf indices (one per tree) for each input sample
     :return: A list of output variables which will be passed to next stage
     """
     operator_name = _get_lightgbm_operator_name(model)
     this_operator = scope.declare_local_operator(operator_name, model)
     this_operator.split = split
+    this_operator.decision_leaf = decision_leaf
     this_operator.inputs = inputs
 
     if operator_name == "LgbmClassifier":
@@ -133,13 +136,21 @@ def _parse_lightgbm_simple_model(scope, model, inputs, split=None):
         # We assume that all scikit-learn operator can only produce a single float tensor.
         variable = scope.declare_local_variable("variable", FloatTensorType())
         this_operator.outputs.append(variable)
+    if decision_leaf:
+        leaf_indices_variable = scope.declare_local_variable(
+            "leaf_indices", Int64Type()
+        )
+        this_operator.outputs.append(leaf_indices_variable)
     return this_operator.outputs
 
 
-def _parse_sklearn_classifier(scope, model, inputs, zipmap=True):
-    probability_tensor = _parse_lightgbm_simple_model(scope, model, inputs)
+def _parse_sklearn_classifier(scope, model, inputs, zipmap=True, decision_leaf=False):
+    probability_tensor = _parse_lightgbm_simple_model(
+        scope, model, inputs, decision_leaf=decision_leaf
+    )
     this_operator = scope.declare_local_operator("LgbmZipMap")
-    this_operator.inputs = probability_tensor
+    # ZipMap only needs label and probabilities (first two outputs)
+    this_operator.inputs = probability_tensor[:2]
     this_operator.zipmap = zipmap
 
     classes = model.classes_
@@ -179,10 +190,14 @@ def _parse_sklearn_classifier(scope, model, inputs, zipmap=True):
         )
     this_operator.outputs.append(output_label)
     this_operator.outputs.append(output_probability)
-    return this_operator.outputs
+    result = list(this_operator.outputs)
+    if decision_leaf:
+        # Pass through the leaf indices variable from the LgbmClassifier operator
+        result.append(probability_tensor[2])
+    return result
 
 
-def _parse_lightgbm(scope, model, inputs, zipmap=True, split=None):
+def _parse_lightgbm(scope, model, inputs, zipmap=True, split=None, decision_leaf=False):
     """
     This is a delegate function. It doesn't nothing but
     invoke the correct parsing function according to the input
@@ -194,13 +209,21 @@ def _parse_lightgbm(scope, model, inputs, zipmap=True, split=None):
     :param zipmap: add operator ZipMap after operator TreeEnsembleClassifier
     :param split: split TreeEnsembleRegressor into multiple node to reduce
         discrepancies
+    :param decision_leaf: if True, an additional output is added to return
+        the leaf indices (one per tree) for each input sample
     :return: The output variables produced by the input model
     """
     if isinstance(model, LGBMClassifier):
-        return _parse_sklearn_classifier(scope, model, inputs, zipmap=zipmap)
+        return _parse_sklearn_classifier(
+            scope, model, inputs, zipmap=zipmap, decision_leaf=decision_leaf
+        )
     if isinstance(model, WrappedBooster) and model.operator_name == "LgbmClassifier":
-        return _parse_sklearn_classifier(scope, model, inputs, zipmap=zipmap)
-    return _parse_lightgbm_simple_model(scope, model, inputs, split=split)
+        return _parse_sklearn_classifier(
+            scope, model, inputs, zipmap=zipmap, decision_leaf=decision_leaf
+        )
+    return _parse_lightgbm_simple_model(
+        scope, model, inputs, split=split, decision_leaf=decision_leaf
+    )
 
 
 def parse_lightgbm(
@@ -211,6 +234,7 @@ def parse_lightgbm(
     custom_shape_calculators=None,
     zipmap=True,
     split=None,
+    decision_leaf=False,
 ):
     raw_model_container = LightGbmModelContainer(model)
     topology = Topology(
@@ -230,7 +254,9 @@ def parse_lightgbm(
     for variable in inputs:
         raw_model_container.add_input(variable)
 
-    outputs = _parse_lightgbm(scope, model, inputs, zipmap=zipmap, split=split)
+    outputs = _parse_lightgbm(
+        scope, model, inputs, zipmap=zipmap, split=split, decision_leaf=decision_leaf
+    )
 
     for variable in outputs:
         raw_model_container.add_output(variable)
diff --git a/onnxmltools/convert/lightgbm/convert.py b/onnxmltools/convert/lightgbm/convert.py
index 83799cbde..e02775fd0 100644
--- a/onnxmltools/convert/lightgbm/convert.py
+++ b/onnxmltools/convert/lightgbm/convert.py
@@ -24,6 +24,7 @@ def convert(
     without_onnx_ml=False,
     zipmap=True,
     split=None,
+    decision_leaf=False,
 ):
     """
     This function produces an equivalent ONNX model of the given lightgbm model.
@@ -65,7 +66,14 @@ def convert(
         Parameter *split* is the number of trees per node. It could be possible to
         do the same with TreeEnsembleClassifier. However, the normalization of the
         probabilities significantly reduces the discrepancies.
-    to use ONNX-ML operators as well.
+    :param decision_leaf: if True, an additional output named *leaf_indices* is
+        added to the ONNX model. It returns an int64 tensor of shape [N, n_trees]
+        where each element is the index of the leaf node reached in the corresponding
+        tree for each input sample. This is equivalent to LightGBM's
+        ``predict(X, pred_leaf=True)``.
+    :param without_onnx_ml: set to True to generate a model composed of ONNX
+        operators only (requires hummingbird-ml); set to False (default) to allow
+        ONNX-ML operators as well.
     :return: An ONNX model (type: ModelProto) which is equivalent to the input lightgbm model
     """
     if initial_types is None:
@@ -92,6 +100,7 @@ def convert(
         custom_shape_calculators,
         zipmap=zipmap,
         split=split,
+        decision_leaf=decision_leaf,
     )
     topology.compile()
     onnx_ml_model = convert_topology(
diff --git a/onnxmltools/convert/lightgbm/operator_converters/LightGbm.py b/onnxmltools/convert/lightgbm/operator_converters/LightGbm.py
index d03a50739..fe25b6bb4 100644
--- a/onnxmltools/convert/lightgbm/operator_converters/LightGbm.py
+++ b/onnxmltools/convert/lightgbm/operator_converters/LightGbm.py
@@ -535,6 +535,79 @@ def _split_tree_ensemble_atts(attrs, split):
     return results
 
 
+def _add_decision_leaf_output(scope, operator, container, attrs, n_classes, gbm_text):
+    """
+    Adds an additional ONNX ``TreeEnsembleRegressor`` node that produces leaf
+    indices (one per tree) for each input sample.  The output is cast to int64
+    and written to the last output variable of *operator*.
+
+    :param scope: Scope object
+    :param operator: the operator being converted
+    :param container: the container to add nodes to
+    :param attrs: the attribute dict already populated with nodes_* entries
+    :param n_classes: number of classes / output targets in the main model
+    :param gbm_text: the raw JSON dump of the booster
+    """
+    n_trees = len(gbm_text["tree_info"])
+
+    # Build a TreeEnsembleRegressor that outputs the sequential leaf index
+    # reached in each tree.  One output column per tree (n_targets = n_trees).
+    leaf_attrs = {k: copy.deepcopy(v) for k, v in attrs.items() if k.startswith("nodes_")}
+    leaf_attrs["name"] = operator.full_name + "_leaf"
+    leaf_attrs["n_targets"] = n_trees
+    leaf_attrs["post_transform"] = "NONE"
+    leaf_attrs["aggregate_function"] = "SUM"
+
+    # For each leaf in the original attrs (class_treeids / class_nodeids),
+    # map tree i → output column i and assign a sequential leaf index
+    # within that tree as the weight.
+    leaf_count_per_tree: dict = {}
+    target_treeids = []
+    target_nodeids = []
+    target_ids = []
+    target_weights = []
+
+    for tree_id, node_id in zip(attrs["class_treeids"], attrs["class_nodeids"]):
+        count = leaf_count_per_tree.get(tree_id, 0)
+        target_treeids.append(tree_id)
+        target_nodeids.append(node_id)
+        target_ids.append(tree_id)
+        target_weights.append(float(count))
+        leaf_count_per_tree[tree_id] = count + 1
+
+    leaf_attrs["target_treeids"] = target_treeids
+    leaf_attrs["target_nodeids"] = target_nodeids
+    leaf_attrs["target_ids"] = target_ids
+    leaf_attrs["target_weights"] = target_weights
+
+    # onnx does not support mixed int/float lists
+    update = {}
+    for k, v in leaf_attrs.items():
+        if not isinstance(v, list):
+            continue
+        tps = set(map(type, v))
+        if len(tps) == 2 and tps == {int, float}:
+            update[k] = [float(x) for x in v]
+    leaf_attrs.update(update)
+
+    leaf_output_name = scope.get_unique_variable_name("leaf_output")
+    container.add_node(
+        "TreeEnsembleRegressor",
+        operator.input_full_names,
+        leaf_output_name,
+        op_domain="ai.onnx.ml",
+        **leaf_attrs,
+    )
+    # Cast float output to int64 for the leaf_indices output variable
+    apply_cast(
+        scope,
+        leaf_output_name,
+        operator.outputs[-1].full_name,
+        container,
+        to=TensorProto.INT64,
+    )
+
+
 def convert_lightgbm(scope, operator, container):
     """
     Converters for *lightgbm*.
@@ -624,6 +697,12 @@ def convert_lightgbm(scope, operator, container):
             ]
             attrs[k] = sorted_list
 
+    # Build leaf index output when decision_leaf=True
+    if getattr(operator, "decision_leaf", False):
+        _add_decision_leaf_output(
+            scope, operator, container, attrs, n_classes, gbm_text
+        )
+
     # Create ONNX object
     if objective.startswith("binary") or objective.startswith("multiclass"):
         # Prepare label information for both of TreeEnsembleClassifier
@@ -833,7 +912,7 @@ def convert_lightgbm(scope, operator, container):
             apply_div(
                 scope,
                 [output_name, denominator_name],
-                operator.output_full_names,
+                operator.outputs[0].full_name,
                 container,
                 broadcast=1,
             )
@@ -841,14 +920,14 @@ def convert_lightgbm(scope, operator, container):
             container.add_node(
                 post_transform,
                 output_name,
-                operator.output_full_names,
+                operator.outputs[0].full_name,
                 name=scope.get_unique_operator_name(post_transform),
             )
         else:
             container.add_node(
                 "Identity",
                 output_name,
-                operator.output_full_names,
+                operator.outputs[0].full_name,
                 name=scope.get_unique_operator_name("Identity"),
             )
 
diff --git a/onnxmltools/convert/lightgbm/shape_calculators/Classifier.py b/onnxmltools/convert/lightgbm/shape_calculators/Classifier.py
index 11657fea8..42b740dca 100644
--- a/onnxmltools/convert/lightgbm/shape_calculators/Classifier.py
+++ b/onnxmltools/convert/lightgbm/shape_calculators/Classifier.py
@@ -25,8 +25,13 @@ def calculate_lightgbm_classifier_output_shapes(operator):
 
     Note that the second case is not allowed as long as ZipMap only produces dictionary.
     """
+    op = operator.raw_operator
+    decision_leaf = getattr(operator, "decision_leaf", False)
+    expected_outputs = 3 if decision_leaf else 2
     check_input_and_output_numbers(
-        operator, input_count_range=1, output_count_range=[1, 2]
+        operator,
+        input_count_range=1,
+        output_count_range=[expected_outputs, expected_outputs],
     )
     check_input_and_output_types(
         operator, good_input_types=[FloatTensorType, Int64TensorType]
@@ -36,7 +41,7 @@ def calculate_lightgbm_classifier_output_shapes(operator):
 
     N = operator.inputs[0].type.shape[0]
 
-    class_labels = operator.raw_operator.classes_
+    class_labels = op.classes_
     if all(isinstance(i, np.ndarray) for i in class_labels):
         class_labels = np.concatenate(class_labels)
     if all(isinstance(i, str) for i in class_labels):
@@ -48,6 +53,10 @@ def calculate_lightgbm_classifier_output_shapes(operator):
     else:
         operator.outputs[1].type = FloatTensorType(shape=[N, 1])
 
+    if decision_leaf:
+        n_trees = op.booster_.num_trees()
+        operator.outputs[2].type = Int64TensorType(shape=[N, n_trees])
+
 
 def calculate_lgbm_zipmap(operator):
     check_input_and_output_numbers(operator, output_count_range=2)
diff --git a/onnxmltools/convert/lightgbm/shape_calculators/Ranker.py b/onnxmltools/convert/lightgbm/shape_calculators/Ranker.py
index 67eaf146b..c04e1faa7 100644
--- a/onnxmltools/convert/lightgbm/shape_calculators/Ranker.py
+++ b/onnxmltools/convert/lightgbm/shape_calculators/Ranker.py
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 from ...common._registration import register_shape_calculator
-from ...common.shape_calculator import calculate_linear_regressor_output_shapes
+from .Regressor import calculate_lightgbm_regressor_output_shapes
+
+register_shape_calculator("LgbmRanker", calculate_lightgbm_regressor_output_shapes)
 
-register_shape_calculator("LgbmRanker", calculate_linear_regressor_output_shapes)
diff --git a/onnxmltools/convert/lightgbm/shape_calculators/Regressor.py b/onnxmltools/convert/lightgbm/shape_calculators/Regressor.py
index 232742766..edc423434 100644
--- a/onnxmltools/convert/lightgbm/shape_calculators/Regressor.py
+++ b/onnxmltools/convert/lightgbm/shape_calculators/Regressor.py
@@ -2,5 +2,37 @@
 
 from ...common._registration import register_shape_calculator
 from ...common.shape_calculator import calculate_linear_regressor_output_shapes
+from ...common.data_types import FloatTensorType, Int64TensorType
+
+
+def calculate_lightgbm_regressor_output_shapes(operator):
+    """
+    Allowed input/output patterns are
+        1. [N, C] ---> [N, 1]
+        2. [N, C] ---> [N, 1], [N, n_trees]  (when decision_leaf=True)
+
+    This operator produces a scalar prediction for every example in a batch. If the input batch size is N, the output
+    shape may be [N, 1]. If decision_leaf is True, a second output of shape [N, n_trees] is produced.
+    """
+    decision_leaf = getattr(operator, "decision_leaf", False)
+    if decision_leaf:
+        from ...common.shape_calculator import check_input_and_output_numbers
+
+        check_input_and_output_numbers(
+            operator, input_count_range=1, output_count_range=2
+        )
+        N = operator.inputs[0].type.shape[0]
+        op = operator.raw_operator
+        if hasattr(op, "n_outputs_"):
+            nout = op.n_outputs_
+        else:
+            nout = 1
+        operator.outputs[0].type = FloatTensorType([N, nout])
+        n_trees = op.booster_.num_trees()
+        operator.outputs[1].type = Int64TensorType(shape=[N, n_trees])
+    else:
+        calculate_linear_regressor_output_shapes(operator)
+
+
+register_shape_calculator("LgbmRegressor", calculate_lightgbm_regressor_output_shapes)
 
-register_shape_calculator("LgbmRegressor", calculate_linear_regressor_output_shapes)
diff --git a/onnxmltools/convert/main.py b/onnxmltools/convert/main.py
index 9bed40b79..19c73c0cb 100644
--- a/onnxmltools/convert/main.py
+++ b/onnxmltools/convert/main.py
@@ -213,6 +213,7 @@ def convert_lightgbm(
     without_onnx_ml=False,
     zipmap=True,
     split=None,
+    decision_leaf=False,
 ):
     if targeted_onnx is not None:
         warnings.warn(
@@ -237,6 +238,7 @@ def convert_lightgbm(
         without_onnx_ml,
         zipmap=zipmap,
         split=split,
+        decision_leaf=decision_leaf,
     )
 
 
diff --git a/tests/lightgbm/test_lightgbm_decision_leaf.py b/tests/lightgbm/test_lightgbm_decision_leaf.py
new file mode 100644
index 000000000..b763c507c
--- /dev/null
+++ b/tests/lightgbm/test_lightgbm_decision_leaf.py
@@ -0,0 +1,145 @@
+# SPDX-License-Identifier: Apache-2.0
+
+import unittest
+
+import numpy
+from numpy.testing import assert_array_equal
+from onnx.defs import onnx_opset_version
+from lightgbm import LGBMClassifier, LGBMRegressor
+import onnxruntime
+from onnxmltools.convert.common.data_types import FloatTensorType
+from onnxmltools.convert.common.onnx_ex import DEFAULT_OPSET_NUMBER
+from onnxmltools.convert import convert_lightgbm
+
+TARGET_OPSET = min(DEFAULT_OPSET_NUMBER, onnx_opset_version())
+
+
+class TestDecisionLeaf(unittest.TestCase):
+    def _make_data(self):
+        X = numpy.array(
+            [[0, 1], [1, 1], [2, 0], [1, 2], [1, 5], [6, 2]], dtype=numpy.float32
+        )
+        y_bin = [0, 1, 0, 1, 1, 0]
+        y_reg = [0.1, 1.2, 0.3, 1.4, 1.5, 0.6]
+        return X, y_bin, y_reg
+
+    def test_decision_leaf_binary_classifier(self):
+        X, y, _ = self._make_data()
+        model = LGBMClassifier(
+            n_estimators=3, min_child_samples=1, max_depth=2, num_thread=1
+        )
+        model.fit(X, y)
+        onx = convert_lightgbm(
+            model,
+            "dummy",
+            initial_types=[("X", FloatTensorType([None, X.shape[1]]))],
+            zipmap=False,
+            target_opset=TARGET_OPSET,
+            decision_leaf=True,
+        )
+        # Model must have 3 outputs: label, probabilities, leaf_indices
+        output_names = [o.name for o in onx.graph.output]
+        self.assertEqual(len(output_names), 3)
+
+        sess = onnxruntime.InferenceSession(
+            onx.SerializeToString(), providers=["CPUExecutionProvider"]
+        )
+        results = sess.run(None, {"X": X})
+        self.assertEqual(len(results), 3)
+
+        label, proba, leaf_indices = results
+        # leaf_indices shape: [N, n_trees]
+        self.assertEqual(leaf_indices.shape, (X.shape[0], model.n_estimators_))
+        self.assertEqual(leaf_indices.dtype, numpy.int64)
+
+        # leaf_indices values should be non-negative integers (node IDs in the tree)
+        self.assertTrue((leaf_indices >= 0).all())
+        # Each tree should produce consistent indices (same input → same leaf)
+        leaf_indices2 = sess.run(None, {"X": X})[2]
+        assert_array_equal(leaf_indices, leaf_indices2)
+
+    def test_decision_leaf_multiclass_classifier(self):
+        X = numpy.array(
+            [[0, 1], [1, 1], [2, 0], [1, 2], [0, 3], [3, 0]], dtype=numpy.float32
+        )
+        y = [0, 1, 2, 0, 1, 2]
+        model = LGBMClassifier(
+            n_estimators=3, min_child_samples=1, num_class=3, num_thread=1
+        )
+        model.fit(X, y)
+        n_trees = model.booster_.num_trees()
+        onx = convert_lightgbm(
+            model,
+            "dummy",
+            initial_types=[("X", FloatTensorType([None, X.shape[1]]))],
+            zipmap=False,
+            target_opset=TARGET_OPSET,
+            decision_leaf=True,
+        )
+        self.assertEqual(len(onx.graph.output), 3)
+
+        sess = onnxruntime.InferenceSession(
+            onx.SerializeToString(), providers=["CPUExecutionProvider"]
+        )
+        results = sess.run(None, {"X": X})
+        self.assertEqual(len(results), 3)
+
+        _, _, leaf_indices = results
+        self.assertEqual(leaf_indices.shape, (X.shape[0], n_trees))
+        self.assertEqual(leaf_indices.dtype, numpy.int64)
+        self.assertTrue((leaf_indices >= 0).all())
+
+    def test_decision_leaf_regressor(self):
+        X, _, y = self._make_data()
+        model = LGBMRegressor(
+            n_estimators=3, min_child_samples=1, max_depth=2, num_thread=1
+        )
+        model.fit(X, y)
+        n_trees = model.booster_.num_trees()
+        onx = convert_lightgbm(
+            model,
+            "dummy",
+            initial_types=[("X", FloatTensorType([None, X.shape[1]]))],
+            target_opset=TARGET_OPSET,
+            decision_leaf=True,
+        )
+        # Model must have 2 outputs: prediction, leaf_indices
+        self.assertEqual(len(onx.graph.output), 2)
+
+        sess = onnxruntime.InferenceSession(
+            onx.SerializeToString(), providers=["CPUExecutionProvider"]
+        )
+        results = sess.run(None, {"X": X})
+        self.assertEqual(len(results), 2)
+
+        pred, leaf_indices = results
+        self.assertEqual(leaf_indices.shape, (X.shape[0], n_trees))
+        self.assertEqual(leaf_indices.dtype, numpy.int64)
+        self.assertTrue((leaf_indices >= 0).all())
+
+    def test_decision_leaf_false_keeps_original_outputs(self):
+        """Ensure decision_leaf=False (default) produces the original outputs."""
+        X, y, _ = self._make_data()
+        model = LGBMClassifier(
+            n_estimators=3, min_child_samples=1, max_depth=2, num_thread=1
+        )
+        model.fit(X, y)
+        onx = convert_lightgbm(
+            model,
+            "dummy",
+            initial_types=[("X", FloatTensorType([None, X.shape[1]]))],
+            zipmap=False,
+            target_opset=TARGET_OPSET,
+            decision_leaf=False,
+        )
+        self.assertEqual(len(onx.graph.output), 2)
+
+        sess = onnxruntime.InferenceSession(
+            onx.SerializeToString(), providers=["CPUExecutionProvider"]
+        )
+        results = sess.run(None, {"X": X})
+        self.assertEqual(len(results), 2)
+
+
+if __name__ == "__main__":
+    unittest.main()