diff --git a/src/lightning/pytorch/CHANGELOG.md b/src/lightning/pytorch/CHANGELOG.md
index 22cf899496ed6..058aeae8124ae 100644
--- a/src/lightning/pytorch/CHANGELOG.md
+++ b/src/lightning/pytorch/CHANGELOG.md
@@ -28,6 +28,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed `LightningDataModule.load_from_checkpoint` to restore the datamodule subclass and hyperparameters ([#21478](https://github.com/Lightning-AI/pytorch-lightning/pull/21478))
 
+- Fixed ``RichModelSummary`` model size display formatting ([#21467](https://github.com/Lightning-AI/pytorch-lightning/pull/21467))
 
 - Fixed ``ModelParallelStrategy`` single-file checkpointing when ``torch.compile`` wraps the model so optimizer states no longer raise ``KeyError`` during save ([#21357](https://github.com/Lightning-AI/pytorch-lightning/issues/21357))
 
diff --git a/src/lightning/pytorch/callbacks/rich_model_summary.py b/src/lightning/pytorch/callbacks/rich_model_summary.py
index 2843806ca595a..e287361e7412a 100644
--- a/src/lightning/pytorch/callbacks/rich_model_summary.py
+++ b/src/lightning/pytorch/callbacks/rich_model_summary.py
@@ -17,7 +17,7 @@
 
 from lightning.pytorch.callbacks import ModelSummary
 from lightning.pytorch.utilities.imports import _RICH_AVAILABLE
-from lightning.pytorch.utilities.model_summary import get_human_readable_count
+from lightning.pytorch.utilities.model_summary import get_formatted_model_size, get_human_readable_count
 
 
 class RichModelSummary(ModelSummary):
@@ -105,8 +105,9 @@ def summarize(
         console.print(table)
 
         parameters = []
-        for param in [trainable_parameters, total_parameters - trainable_parameters, total_parameters, model_size]:
+        for param in [trainable_parameters, total_parameters - trainable_parameters, total_parameters]:
             parameters.append("{:<{}}".format(get_human_readable_count(int(param)), 10))
+        parameters.append("{:<{}}".format(get_formatted_model_size(model_size), 10))
 
         grid = Table.grid(expand=True)
         grid.add_column()
diff --git a/tests/tests_pytorch/callbacks/test_rich_model_summary.py b/tests/tests_pytorch/callbacks/test_rich_model_summary.py
index af385bb1a9b39..4fd22e39babc5 100644
--- a/tests/tests_pytorch/callbacks/test_rich_model_summary.py
+++ b/tests/tests_pytorch/callbacks/test_rich_model_summary.py
@@ -70,3 +70,34 @@ def example_input_array(self) -> Any:
     # assert that the input summary data was converted correctly
     args, _ = mock_table_add_row.call_args_list[0]
     assert args[1:] == ("0", "layer", "Linear", "66  ", "train", "512  ", "[4, 32]", "[4, 2]")
+
+
+@RunIf(rich=True)
+def test_rich_summary_model_size_formatting():
+    """Ensure model_size uses get_formatted_model_size, not get_human_readable_count."""
+    from io import StringIO
+
+    from rich.console import Console
+
+    model_summary = RichModelSummary()
+    model = BoringModel()
+    summary = summarize(model)
+    summary_data = summary._get_summary_data()
+
+    output = StringIO()
+    console = Console(file=output, force_terminal=True)
+
+    with mock.patch("rich.get_console", return_value=console):
+        model_summary.summarize(
+            summary_data=summary_data,
+            total_parameters=1,
+            trainable_parameters=1,
+            model_size=5500.0,
+            total_training_modes=summary.total_training_modes,
+            total_flops=1,
+        )
+
+    result = output.getvalue()
+    # model_size=5500.0 should display as "5,500.000" (formatted), not "5.5 K" (human readable count)
+    assert "5,500.000" in result
+    assert "5.5 K" not in result