diff --git a/CHANGELOG.md b/CHANGELOG.md
index 46bb8106bf80b..0126950f2697b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
+- Allow logging to an existing run ID in MLflow with `MLFlowLogger` ([#12290](https://github.com/PyTorchLightning/pytorch-lightning/pull/12290))
+
+
 - Enable gradient accumulation using Horovod's `backward_passes_per_step` ([#11911](https://github.com/PyTorchLightning/pytorch-lightning/pull/11911))
 
 
diff --git a/pytorch_lightning/loggers/mlflow.py b/pytorch_lightning/loggers/mlflow.py
index f43d8358e1d43..a74dda324e15f 100644
--- a/pytorch_lightning/loggers/mlflow.py
+++ b/pytorch_lightning/loggers/mlflow.py
@@ -87,7 +87,7 @@ def any_lightning_module_function_or_hook(self):
                 self.logger.experiment.whatever_ml_flow_supports(...)
 
     Args:
-        experiment_name: The name of the experiment
+        experiment_name: The name of the experiment.
         run_name: Name of the new run. The `run_name` is internally stored as a ``mlflow.runName`` tag.
             If the ``mlflow.runName`` tag has already been set in `tags`, the value is overridden by the `run_name`.
         tracking_uri: Address of local or remote tracking server.
@@ -100,6 +100,7 @@ def any_lightning_module_function_or_hook(self):
         prefix: A string to put at the beginning of metric keys.
         artifact_location: The location to store run artifacts. If not provided, the server picks an appropriate
             default.
+        run_id: The run identifier of the experiment. If not provided, a new run is started.
 
     Raises:
         ModuleNotFoundError:
@@ -117,6 +118,7 @@ def __init__(
         save_dir: Optional[str] = "./mlruns",
         prefix: str = "",
         artifact_location: Optional[str] = None,
+        run_id: Optional[str] = None,
     ):
         if mlflow is None:
             raise ModuleNotFoundError(
@@ -130,11 +132,13 @@ def __init__(
         self._experiment_id = None
         self._tracking_uri = tracking_uri
         self._run_name = run_name
-        self._run_id = None
+        self._run_id = run_id
         self.tags = tags
         self._prefix = prefix
         self._artifact_location = artifact_location
 
+        self._initialized = False
+
         self._mlflow_client = MlflowClient(tracking_uri)
 
     @property
@@ -149,6 +153,16 @@ def experiment(self) -> MlflowClient:
             self.logger.experiment.some_mlflow_function()
 
         """
+
+        if self._initialized:
+            return self._mlflow_client
+
+        if self._run_id is not None:
+            run = self._mlflow_client.get_run(self._run_id)
+            self._experiment_id = run.info.experiment_id
+            self._initialized = True
+            return self._mlflow_client
+
         if self._experiment_id is None:
             expt = self._mlflow_client.get_experiment_by_name(self._experiment_name)
             if expt is not None:
@@ -169,6 +183,7 @@ def experiment(self) -> MlflowClient:
                 self.tags[MLFLOW_RUN_NAME] = self._run_name
             run = self._mlflow_client.create_run(experiment_id=self._experiment_id, tags=resolve_tags(self.tags))
             self._run_id = run.info.run_id
+        self._initialized = True
         return self._mlflow_client
 
     @property
diff --git a/tests/loggers/test_mlflow.py b/tests/loggers/test_mlflow.py
index 77afe361b035f..1e9e8ec271bd5 100644
--- a/tests/loggers/test_mlflow.py
+++ b/tests/loggers/test_mlflow.py
@@ -40,6 +40,7 @@ def test_mlflow_logger_exists(client, mlflow, tmpdir):
 
     run1 = MagicMock()
     run1.info.run_id = "run-id-1"
+    run1.info.experiment_id = "exp-id-1"
 
     run2 = MagicMock()
     run2.info.run_id = "run-id-2"
@@ -113,6 +114,27 @@ def test_mlflow_run_name_setting(client, mlflow, tmpdir):
     client.return_value.create_run.assert_called_with(experiment_id="exp-id", tags=default_tags)
 
 
+@mock.patch("pytorch_lightning.loggers.mlflow.mlflow")
+@mock.patch("pytorch_lightning.loggers.mlflow.MlflowClient")
+def test_mlflow_run_id_setting(client, mlflow, tmpdir):
+    """Test that the run_id argument uses the provided run_id."""
+
+    run = MagicMock()
+    run.info.run_id = "run-id"
+    run.info.experiment_id = "experiment-id"
+
+    # simulate existing run
+    client.return_value.get_run = MagicMock(return_value=run)
+
+    # run_id exists uses the existing run
+    logger = MLFlowLogger("test", run_id=run.info.run_id, save_dir=tmpdir)
+    _ = logger.experiment
+    client.return_value.get_run.assert_called_with(run.info.run_id)
+    assert logger.experiment_id == run.info.experiment_id
+    assert logger.run_id == run.info.run_id
+    client.reset_mock(return_value=True)
+
+
 @mock.patch("pytorch_lightning.loggers.mlflow.mlflow")
 @mock.patch("pytorch_lightning.loggers.mlflow.MlflowClient")
 def test_mlflow_log_dir(client, mlflow, tmpdir):