diff --git a/orttraining/orttraining/test/training_ops/cpu/nn/pool_gradient_op_test.cc b/orttraining/orttraining/test/training_ops/cpu/nn/pool_gradient_op_test.cc
new file mode 100644
index 0000000000000..bf10ea01a8152
--- /dev/null
+++ b/orttraining/orttraining/test/training_ops/cpu/nn/pool_gradient_op_test.cc
@@ -0,0 +1,48 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#include "gtest/gtest.h"
+#include "test/providers/provider_test_utils.h"
+#include "test/util/include/default_providers.h"
+
+namespace onnxruntime {
+namespace test {
+
+// Verify that MaxPoolGrad rejects indices that exceed the output buffer size.
+TEST(MaxPoolGradTest, IndicesOutOfRange) {
+  OpTester test("MaxPoolGrad", 9, kOnnxDomain);
+
+  // dY: shape [1, 1, 2, 2]
+  test.AddInput<float>("dY", {1, 1, 2, 2}, {1.0f, 1.0f, 1.0f, 1.0f});
+  // Indices: same shape, last value 100 is out of range [0, 9)
+  test.AddInput<int64_t>("Indices", {1, 1, 2, 2}, {0, 1, 2, 100});
+  // Expected dX: shape [1, 1, 3, 3] → 9 elements
+  test.AddOutput<float>("dX", {1, 1, 3, 3}, {0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f});
+
+  std::vector<std::unique_ptr<IExecutionProvider>> execution_providers;
+  execution_providers.push_back(DefaultCpuExecutionProvider());
+  test.Run(OpTester::ExpectResult::kExpectFailure,
+           "Invalid index in MaxPoolGrad: index value 100 is out of range [0, 9).",
+           {}, nullptr, &execution_providers);
+}
+
+// Verify that MaxPoolGrad rejects negative indices.
+TEST(MaxPoolGradTest, IndicesNegative) {
+  OpTester test("MaxPoolGrad", 9, kOnnxDomain);
+
+  // dY: shape [1, 1, 2, 2]
+  test.AddInput<float>("dY", {1, 1, 2, 2}, {1.0f, 1.0f, 1.0f, 1.0f});
+  // Indices: same shape, value -1 is negative and out of range
+  test.AddInput<int64_t>("Indices", {1, 1, 2, 2}, {0, 1, -1, 3});
+  // Expected dX: shape [1, 1, 3, 3] → 9 elements
+  test.AddOutput<float>("dX", {1, 1, 3, 3}, {0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f});
+
+  std::vector<std::unique_ptr<IExecutionProvider>> execution_providers;
+  execution_providers.push_back(DefaultCpuExecutionProvider());
+  test.Run(OpTester::ExpectResult::kExpectFailure,
+           "Invalid index in MaxPoolGrad: index value -1 is out of range [0, 9).",
+           {}, nullptr, &execution_providers);
+}
+
+}  // namespace test
+}  // namespace onnxruntime
diff --git a/orttraining/orttraining/training_ops/cpu/nn/pool_gradient_op.cc b/orttraining/orttraining/training_ops/cpu/nn/pool_gradient_op.cc
index 769b4d1bc2bd8..1c384c2397ab1 100644
--- a/orttraining/orttraining/training_ops/cpu/nn/pool_gradient_op.cc
+++ b/orttraining/orttraining/training_ops/cpu/nn/pool_gradient_op.cc
@@ -54,9 +54,15 @@ Status MaxPoolGrad<T>::Compute(OpKernelContext* context) const {
 
   EigenVectorMap<T>(dX_data, narrow<Eigen::Index>(dX_shape.Size())).setZero();
 
+  const int64_t dX_size = dX_shape.Size();
   for (int64_t i = 0; i < dY->Shape().Size(); ++i) {
-    T* p_dX_data = dX_data + indices_data[i];
-    *p_dX_data += dY_data[i];
+    int64_t index = indices_data[i];
+    if (index < 0 || index >= dX_size) {
+      return ORT_MAKE_STATUS(ONNXRUNTIME, INVALID_ARGUMENT,
+                             "Invalid index in MaxPoolGrad: index value ", index,
+                             " is out of range [0, ", dX_size, ").");
+    }
+    dX_data[index] += dY_data[i];
   }
 
   return Status::OK();