diff --git a/src/lightning/pytorch/CHANGELOG.md b/src/lightning/pytorch/CHANGELOG.md
index 176e34273d776..96fcf3daa6396 100644
--- a/src/lightning/pytorch/CHANGELOG.md
+++ b/src/lightning/pytorch/CHANGELOG.md
@@ -16,6 +16,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Added Torch-Tensorrt integration with `LightningModule` ([#20808](https://github.com/Lightning-AI/pytorch-lightning/pull/20808))
 
 
+- Added `PossibleUserWarning` that is raised if modules are in eval mode when training starts ([#21146](https://github.com/Lightning-AI/pytorch-lightning/pull/21146))
+
 ### Changed
 
 - Default to `RichProgressBar` and `RichModelSummary` if the rich package is available. Fallback to TQDMProgressBar and ModelSummary otherwise. ([#9580](https://github.com/Lightning-AI/pytorch-lightning/pull/9580))
diff --git a/src/lightning/pytorch/loops/fit_loop.py b/src/lightning/pytorch/loops/fit_loop.py
index 31d6724a043a3..f25c33359a78a 100644
--- a/src/lightning/pytorch/loops/fit_loop.py
+++ b/src/lightning/pytorch/loops/fit_loop.py
@@ -414,6 +414,9 @@ def on_run_start(self) -> None:
             self.epoch_loop.val_loop.setup_data()
             trainer.training = True
 
+        # Check for modules in eval mode at training start
+        self._warn_if_modules_in_eval_mode()
+
         call._call_callback_hooks(trainer, "on_train_start")
         call._call_lightning_module_hook(trainer, "on_train_start")
         call._call_strategy_hook(trainer, "on_train_start")
@@ -515,6 +518,19 @@ def on_load_checkpoint(self, state_dict: dict) -> None:
         self._combined_loader_states_to_load = state_dict.get("combined_loader", [])
         super().on_load_checkpoint(state_dict)
 
+    def _warn_if_modules_in_eval_mode(self) -> None:
+        """Warn if any modules are in eval mode at the start of training."""
+        model = self.trainer.lightning_module
+        eval_modules = [name for name, module in model.named_modules() if not module.training]
+
+        if eval_modules:
+            rank_zero_warn(
+                f"Found {len(eval_modules)} module(s) in eval mode at the start of training."
+                " This may lead to unexpected behavior during training. If this is intentional,"
+                " you can ignore this warning.",
+                category=PossibleUserWarning,
+            )
+
     def _should_accumulate(self) -> bool:
         """Whether the gradients should be accumulated."""
         return self.epoch_loop._should_accumulate()
diff --git a/tests/tests_pytorch/loops/test_training_loop.py b/tests/tests_pytorch/loops/test_training_loop.py
index e3a4c37f6a284..f5aaa18095fc5 100644
--- a/tests/tests_pytorch/loops/test_training_loop.py
+++ b/tests/tests_pytorch/loops/test_training_loop.py
@@ -13,12 +13,14 @@
 # limitations under the License.
 import itertools
 import logging
+import warnings
 from unittest.mock import Mock
 
 import pytest
 import torch
 from torch.utils.data import DataLoader
 
+from lightning.fabric.utilities.warnings import PossibleUserWarning
 from lightning.pytorch import Trainer, seed_everything
 from lightning.pytorch.demos.boring_classes import BoringModel
 from lightning.pytorch.loops import _FitLoop
@@ -277,3 +279,29 @@ def __iter__(self):
 
     # assert progress bar callback uses correct total steps
     assert pbar.train_progress_bar.total == max_steps
+
+
+@pytest.mark.parametrize("warn", [True, False])
+def test_eval_mode_warning(tmp_path, warn):
+    """Test that a warning is raised if any module is in eval mode at the start of training."""
+    model = BoringModel()
+    if warn:
+        model.some_eval_module = torch.nn.Linear(32, 16)
+        model.some_eval_module.eval()
+
+    trainer = Trainer(
+        default_root_dir=tmp_path,
+        max_epochs=1,
+    )
+
+    if warn:
+        with pytest.warns(PossibleUserWarning):
+            trainer.fit(model)
+    else:
+        with warnings.catch_warnings(record=True) as warning_list:
+            warnings.simplefilter("always")
+            trainer.fit(model)
+            eval_warnings = [
+                w for w in warning_list if issubclass(w.category, PossibleUserWarning) and "eval mode" in str(w.message)
+            ]
+            assert len(eval_warnings) == 0, "Expected no eval mode warnings"