diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5e790c38cfcd6..384fb6a20e1a0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -153,6 +153,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed truncated backprop through time enablement when set as a property on the LightningModule and not the Trainer ([#8804](https://github.com/PyTorchLightning/pytorch-lightning/pull/8804/))
 
 
+- Fixed plateau scheduler stepping on incomplete epoch ([#8861](https://github.com/PyTorchLightning/pytorch-lightning/pull/8861))
+
+
 ## [1.4.0] - 2021-07-27
 
 ### Added
diff --git a/pytorch_lightning/loops/epoch/training_epoch_loop.py b/pytorch_lightning/loops/epoch/training_epoch_loop.py
index 52b31b67edfa9..213be0de21e6d 100644
--- a/pytorch_lightning/loops/epoch/training_epoch_loop.py
+++ b/pytorch_lightning/loops/epoch/training_epoch_loop.py
@@ -227,7 +227,8 @@ def on_run_end(self) -> List[List[STEP_OUTPUT]]:
         self.trainer.call_hook("on_epoch_end")
         self.trainer.logger_connector.on_epoch_end()
 
-        self.update_lr_schedulers("epoch", update_plateau_schedulers=True)
+        if self._num_training_batches_reached(self.is_last_batch):
+            self.update_lr_schedulers("epoch", update_plateau_schedulers=True)
 
         epoch_output = self._epoch_output
         # free memory
diff --git a/tests/trainer/optimization/test_optimizers.py b/tests/trainer/optimization/test_optimizers.py
index c0541f1c142b3..3c8a3d5ae8e68 100644
--- a/tests/trainer/optimization/test_optimizers.py
+++ b/tests/trainer/optimization/test_optimizers.py
@@ -20,7 +20,6 @@
 from pytorch_lightning import Callback, Trainer
 from pytorch_lightning.callbacks import ModelCheckpoint
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
-from tests.base import EvalModelTemplate
 from tests.helpers.boring_model import BoringModel
 from tests.helpers.runif import RunIf
 
@@ -79,7 +78,7 @@ def test_reducelronplateau_with_no_monitor_raises(tmpdir):
     """
     Test exception when a ReduceLROnPlateau is used with no monitor
     """
-    model = EvalModelTemplate()
+    model = BoringModel()
     optimizer = optim.Adam(model.parameters())
     model.configure_optimizers = lambda: ([optimizer], [optim.lr_scheduler.ReduceLROnPlateau(optimizer)])
     trainer = Trainer(default_root_dir=tmpdir, fast_dev_run=True)
@@ -93,7 +92,7 @@ def test_reducelronplateau_with_no_monitor_in_lr_scheduler_dict_raises(tmpdir):
     """
     Test exception when lr_scheduler dict has a ReduceLROnPlateau with no monitor
     """
-    model = EvalModelTemplate()
+    model = BoringModel()
     optimizer = optim.Adam(model.parameters())
     model.configure_optimizers = lambda: {
         "optimizer": optimizer,
@@ -376,33 +375,47 @@ def configure_optimizers(self):
     trainer.fit(model)
 
 
-def test_lr_scheduler_strict(tmpdir):
+@pytest.mark.parametrize("complete_epoch", [True, False])
+@mock.patch("torch.optim.lr_scheduler.ReduceLROnPlateau.step")
+def test_lr_scheduler_strict(step_mock, tmpdir, complete_epoch):
     """
     Test "strict" support in lr_scheduler dict
     """
-    model = EvalModelTemplate()
+    model = BoringModel()
     optimizer = optim.Adam(model.parameters())
     scheduler = optim.lr_scheduler.ReduceLROnPlateau(optimizer)
-    trainer = Trainer(default_root_dir=tmpdir, max_epochs=1)
+    max_epochs = 1 if complete_epoch else None
+    max_steps = None if complete_epoch else 1
+    trainer = Trainer(default_root_dir=tmpdir, max_epochs=max_epochs, max_steps=max_steps)
 
     model.configure_optimizers = lambda: {
         "optimizer": optimizer,
         "lr_scheduler": {"scheduler": scheduler, "monitor": "giraffe", "strict": True},
     }
-    with pytest.raises(
-        MisconfigurationException,
-        match=r"ReduceLROnPlateau conditioned on metric .* which is not available\. Available metrics are:",
-    ):
+
+    if complete_epoch:
+        with pytest.raises(
+            MisconfigurationException,
+            match=r"ReduceLROnPlateau conditioned on metric .* which is not available\. Available metrics are:",
+        ):
+            trainer.fit(model)
+    else:
         trainer.fit(model)
 
+    step_mock.assert_not_called()
+
     model.configure_optimizers = lambda: {
         "optimizer": optimizer,
         "lr_scheduler": {"scheduler": scheduler, "monitor": "giraffe", "strict": False},
     }
-    with pytest.warns(
-        RuntimeWarning, match=r"ReduceLROnPlateau conditioned on metric .* which is not available but strict"
-    ):
-        trainer.fit(model)
+
+    if complete_epoch:
+        with pytest.warns(
+            RuntimeWarning, match=r"ReduceLROnPlateau conditioned on metric .* which is not available but strict"
+        ):
+            trainer.fit(model)
+
+    step_mock.assert_not_called()
 
 
 def test_unknown_configure_optimizers_raises(tmpdir):