diff --git a/monai/apps/nnunet/nnunetv2_runner.py b/monai/apps/nnunet/nnunetv2_runner.py
index 8a10849904..ac75d96d94 100644
--- a/monai/apps/nnunet/nnunetv2_runner.py
+++ b/monai/apps/nnunet/nnunetv2_runner.py
@@ -525,35 +525,70 @@ def train_single_model(self, config: Any, fold: int, gpu_id: tuple | list | int
             kwargs.pop("npz")
             logger.warning("please specify the `export_validation_probabilities` in the __init__ of `nnUNetV2Runner`.")
 
-        cmd = self.train_single_model_command(config, fold, gpu_id, kwargs)
-        run_cmd(cmd, shell=True)
+        cmd, env = self.train_single_model_command(config, fold, gpu_id, kwargs)
+        run_cmd(cmd, env=env)
 
-    def train_single_model_command(self, config, fold, gpu_id, kwargs):
-        if isinstance(gpu_id, (tuple, list)):
+    def train_single_model_command(
+        self, config: str, fold: int, gpu_id: int | str | tuple | list, kwargs: dict[str, Any]
+    ) -> tuple[list[str], dict[str, str]]:
+        """
+        Build the shell command string for training a single nnU-Net model.
+
+        Args:
+            config: Configuration name (e.g., "3d_fullres").
+            fold: Cross-validation fold index (0-4).
+            gpu_id: Device selector—int, str (MIG UUID), or tuple/list for multi-GPU.
+            kwargs: Additional CLI arguments forwarded to nnUNetv2_train.
+
+        Returns:
+            Shell command string.
+
+        Raises:
+            ValueError: If gpu_id is an empty tuple or list.
+        """
+        env = os.environ.copy()
+        device_setting: str | None = None
+        num_gpus = 1
+        if isinstance(gpu_id, str):
+            device_setting = gpu_id
+            num_gpus = 1
+        elif isinstance(gpu_id, (tuple, list)):
+            if len(gpu_id) == 0:
+                raise ValueError("gpu_id tuple/list cannot be empty")
             if len(gpu_id) > 1:
-                gpu_ids_str = ""
-                for _i in range(len(gpu_id)):
-                    gpu_ids_str += f"{gpu_id[_i]},"
-                device_setting = f"CUDA_VISIBLE_DEVICES={gpu_ids_str[:-1]}"
-            else:
-                device_setting = f"CUDA_VISIBLE_DEVICES={gpu_id[0]}"
+                device_setting = ",".join(str(x) for x in gpu_id)
+                num_gpus = len(gpu_id)
+            elif len(gpu_id) == 1:
+                device_setting = str(gpu_id[0])
+                num_gpus = 1
         else:
-            device_setting = f"CUDA_VISIBLE_DEVICES={gpu_id}"
-        num_gpus = 1 if isinstance(gpu_id, int) or len(gpu_id) == 1 else len(gpu_id)
-
-        cmd = (
-            f"{device_setting} nnUNetv2_train "
-            + f"{self.dataset_name_or_id} {config} {fold} "
-            + f"-tr {self.trainer_class_name} -num_gpus {num_gpus}"
-        )
+            device_setting = str(gpu_id)
+            num_gpus = 1
+        env_cuda = env.get("CUDA_VISIBLE_DEVICES")
+        if env_cuda is not None and device_setting == "0":
+            logger.info(f"Using existing environment variable CUDA_VISIBLE_DEVICES='{env_cuda}'")
+            device_setting = None
+        elif device_setting is not None:
+            env["CUDA_VISIBLE_DEVICES"] = device_setting
+
+        cmd = [
+            "nnUNetv2_train",
+            f"{self.dataset_name_or_id}",
+            f"{config}",
+            f"{fold}",
+            "-tr",
+            f"{self.trainer_class_name}",
+            "-num_gpus",
+            f"{num_gpus}",
+        ]
         if self.export_validation_probabilities:
-            cmd += " --npz"
+            cmd.append("--npz")
         for _key, _value in kwargs.items():
             if _key == "p" or _key == "pretrained_weights":
-                cmd += f" -{_key} {_value}"
+                cmd.extend([f"-{_key}", f"{_value}"])
             else:
-                cmd += f" --{_key} {_value}"
-        return cmd
+                cmd.extend([f"--{_key}", f"{_value}"])
+        return cmd, env
 
     def train(
         self,
@@ -779,7 +814,7 @@ def predict(
         part_id: int = 0,
         num_processes_preprocessing: int = -1,
         num_processes_segmentation_export: int = -1,
-        gpu_id: int = 0,
+        gpu_id: int | str = 0,
     ) -> None:
         """
         Use this to run inference with nnU-Net. This function is used when you want to manually specify a folder containing
@@ -813,9 +848,14 @@ def predict(
             num_processes_preprocessing: out-of-RAM issues.
             num_processes_segmentation_export: Number of processes used for segmentation export.
                 More is not always better. Beware of out-of-RAM issues.
-            gpu_id: which GPU to use for prediction.
+            gpu_id: GPU device index (int) or MIG UUID (str) for prediction.
+                If CUDA_VISIBLE_DEVICES is already set and gpu_id is 0, the existing
+                environment variable is preserved.
         """
-        os.environ["CUDA_VISIBLE_DEVICES"] = f"{gpu_id}"
+        if "CUDA_VISIBLE_DEVICES" in os.environ and (gpu_id == 0 or gpu_id == "0"):
+            logger.info(f"Predict: Using existing CUDA_VISIBLE_DEVICES={os.environ['CUDA_VISIBLE_DEVICES']}")
+        else:
+            os.environ["CUDA_VISIBLE_DEVICES"] = f"{gpu_id}"
 
         from nnunetv2.inference.predict_from_raw_data import nnUNetPredictor