diff --git a/sagemaker-train/src/sagemaker/train/tuner.py b/sagemaker-train/src/sagemaker/train/tuner.py
index 0931ae1816..54eb65feea 100644
--- a/sagemaker-train/src/sagemaker/train/tuner.py
+++ b/sagemaker-train/src/sagemaker/train/tuner.py
@@ -1413,6 +1413,7 @@ def _build_training_job_definition(self, inputs):
             DataSource,
             S3DataSource,
         )
+        from sagemaker.core.utils.utils import Unassigned
 
         model_trainer = self.model_trainer
 
@@ -1520,6 +1521,16 @@ def _build_training_job_definition(self, inputs):
             ),
         )
 
+        # Preserve the volume KMS key configured on the ModelTrainer's compute so that
+        # HPO child training jobs encrypt their storage volumes with the customer-managed
+        # key. Omit the field entirely when no key is configured; leaving it Unassigned
+        # keeps it out of the serialized request (issue #6521). A PipelineVariable or a
+        # plain string is passed through unchanged.
+        if model_trainer.compute:
+            volume_kms_key_id = model_trainer.compute.volume_kms_key_id
+            if volume_kms_key_id is not None and not isinstance(volume_kms_key_id, Unassigned):
+                resource_config.volume_kms_key_id = volume_kms_key_id
+
         # Build stopping condition
         stopping_condition = StoppingCondition()
         if model_trainer.stopping_condition:
diff --git a/sagemaker-train/tests/unit/train/test_tuner.py b/sagemaker-train/tests/unit/train/test_tuner.py
index ff4800be51..f502fbbb26 100644
--- a/sagemaker-train/tests/unit/train/test_tuner.py
+++ b/sagemaker-train/tests/unit/train/test_tuner.py
@@ -70,6 +70,7 @@ def _create_mock_model_trainer(with_internal_channels=False, with_spot_training=
     trainer.compute.instance_type = "ml.m5.xlarge"
     trainer.compute.instance_count = 1
     trainer.compute.volume_size_in_gb = 30
+    trainer.compute.volume_kms_key_id = None
     trainer.stopping_condition = MagicMock()
     trainer.stopping_condition.max_runtime_in_seconds = 3600
     trainer.input_data_config = None
@@ -865,6 +866,66 @@ def test_omitted_values_stay_unassigned(self):
         assert not isinstance(definition.retry_strategy, RetryStrategy)
 
 
+class TestBuildTrainingJobDefinitionVolumeKmsKey:
+    """Regression for #6521.
+
+    ``_build_training_job_definition`` dropped ``volume_kms_key_id`` from
+    ``ModelTrainer.compute``, so HPO child training jobs silently failed to
+    encrypt their storage volumes with the configured customer-managed KMS key.
+    """
+
+    @staticmethod
+    def _tuner(trainer):
+        return HyperparameterTuner(
+            model_trainer=trainer,
+            objective_metric_name="accuracy",
+            hyperparameter_ranges=_create_single_hp_range(),
+        )
+
+    def test_explicit_key_is_propagated(self):
+        trainer = _create_mock_model_trainer()
+        trainer.compute.volume_kms_key_id = (
+            "arn:aws:kms:us-east-2:123456789012:key/00000000-0000-0000-0000-000000000000"
+        )
+
+        definition = self._tuner(trainer)._build_training_job_definition(None)
+
+        assert definition.resource_config.volume_kms_key_id == (
+            "arn:aws:kms:us-east-2:123456789012:key/00000000-0000-0000-0000-000000000000"
+        )
+
+    def test_plain_string_key_is_propagated(self):
+        trainer = _create_mock_model_trainer()
+        trainer.compute.volume_kms_key_id = "1234abcd-12ab-34cd-56ef-1234567890ab"
+
+        definition = self._tuner(trainer)._build_training_job_definition(None)
+
+        assert (
+            definition.resource_config.volume_kms_key_id == "1234abcd-12ab-34cd-56ef-1234567890ab"
+        )
+
+    def test_pipeline_variable_key_is_propagated(self):
+        trainer = _create_mock_model_trainer()
+        kms_param = ParameterString(name="VolumeKmsKeyId")
+        trainer.compute.volume_kms_key_id = kms_param
+
+        definition = self._tuner(trainer)._build_training_job_definition(None)
+
+        assert definition.resource_config.volume_kms_key_id is kms_param
+
+    def test_omitted_key_stays_unassigned(self):
+        from sagemaker.core.utils.utils import Unassigned
+
+        trainer = _create_mock_model_trainer()
+        trainer.compute.volume_kms_key_id = None
+
+        definition = self._tuner(trainer)._build_training_job_definition(None)
+
+        # Leaving the field Unassigned keeps VolumeKmsKeyId out of the
+        # serialized CreateHyperParameterTuningJob request.
+        assert isinstance(definition.resource_config.volume_kms_key_id, Unassigned)
+
+
 class TestHyperparameterTunerTagInheritance:
     """Test tag inheritance from ModelTrainer (issue #6472)."""
 
diff --git a/sagemaker-train/tests/unit/train/test_tuner_driver_channels.py b/sagemaker-train/tests/unit/train/test_tuner_driver_channels.py
index d939611bc9..97b747dfd5 100644
--- a/sagemaker-train/tests/unit/train/test_tuner_driver_channels.py
+++ b/sagemaker-train/tests/unit/train/test_tuner_driver_channels.py
@@ -79,6 +79,7 @@ def _mock_model_trainer(**overrides):
     trainer.compute.instance_type = "ml.m5.xlarge"
     trainer.compute.instance_count = 1
     trainer.compute.volume_size_in_gb = 30
+    trainer.compute.volume_kms_key_id = None
     trainer.stopping_condition = MagicMock()
     trainer.stopping_condition.max_runtime_in_seconds = 3600
     trainer.input_data_config = None