diff --git a/CHANGELOG.md b/CHANGELOG.md
index 221f4ff8ee..a280660229 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -61,10 +61,13 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Removed the deprecated `outputs` argument in both the `LightningModule.on_train_epoch_end` and `Callback.on_train_epoch_end` hooks ([#8587](https://github.com/PyTorchLightning/pytorch-lightning/pull/8587))
 
 
+
 - Delete the deprecated `TrainerLoggingMixin` class ([#8609](https://github.com/PyTorchLightning/pytorch-lightning/pull/8609))
 
 
--
+
+- Removed the deprecated `optimizer_idx` from `training_step` as an accepted argument in manual optimization ([#8576](https://github.com/PyTorchLightning/pytorch-lightning/pull/8576))
+
 
 
 -
diff --git a/pytorch_lightning/loops/batch/training_batch_loop.py b/pytorch_lightning/loops/batch/training_batch_loop.py
index 8a129bbacf..152d34cbb0 100644
--- a/pytorch_lightning/loops/batch/training_batch_loop.py
+++ b/pytorch_lightning/loops/batch/training_batch_loop.py
@@ -160,19 +160,20 @@ class TrainingBatchLoop(Loop):
         return len(self.get_active_optimizers(batch_idx))
 
     def _run_optimization(
-        self, batch_idx: int, split_batch: Any, opt_idx: int = 0, optimizer: Optional[torch.optim.Optimizer] = None
+        self,
+        batch_idx: int,
+        split_batch: Any,
+        opt_idx: Optional[int] = None,
+        optimizer: Optional[torch.optim.Optimizer] = None,
     ):
         """Runs closure (train step + backward) together with optimization if necessary.
 
         Args:
             batch_idx: the index of the current batch
             split_batch: the current tbptt split of the whole batch
-            opt_idx: the index of the current optimizer
-            optimizer: the current optimizer
+            opt_idx: the index of the current optimizer or `None` in case of manual optimization
+            optimizer: the current optimizer or `None` in case of manual optimization
         """
-        # TODO(@awaelchli): In v1.5, when optimizer_idx gets removed from training_step in manual_optimization, change
-        #   opt_idx=0 to opt_idx=None in the signature here
-
         # toggle model params
         self._run_optimization_start(opt_idx, optimizer)
 
@@ -625,10 +626,10 @@ class TrainingBatchLoop(Loop):
             has_opt_idx_in_train_step = is_param_in_hook_signature(training_step_fx, "optimizer_idx")
             if has_opt_idx_in_train_step:
                 if not lightning_module.automatic_optimization:
-                    self._warning_cache.deprecation(
-                        "`training_step` hook signature has changed in v1.3."
-                        " `optimizer_idx` argument has been removed in case of manual optimization. Support for"
-                        " the old signature will be removed in v1.5"
+                    raise ValueError(
+                        "Your `LightningModule.training_step` signature contains an `optimizer_idx` argument but"
+                        " in manual optimization optimizers must be handled by the user. Remove the optimizer_idx"
+                        " argument or set `self.automatic_optimization = True`."
                     )
                 step_kwargs["optimizer_idx"] = opt_idx
             elif not has_opt_idx_in_train_step and lightning_module.automatic_optimization:
diff --git a/tests/deprecated_api/test_remove_1-5.py b/tests/deprecated_api/test_remove_1-5.py
index cce9176c97..8dbe17e7a0 100644
--- a/tests/deprecated_api/test_remove_1-5.py
+++ b/tests/deprecated_api/test_remove_1-5.py
@@ -166,26 +166,6 @@ def test_v1_5_0_running_sanity_check():
         assert not trainer.running_sanity_check
 
 
-def test_old_training_step_signature_with_opt_idx_manual_opt(tmpdir):
-    class OldSignatureModel(BoringModel):
-        def __init__(self):
-            super().__init__()
-            self.automatic_optimization = False
-
-        def training_step(self, batch, batch_idx, optimizer_idx):
-            assert optimizer_idx == 0
-            return super().training_step(batch, batch_idx)
-
-        def configure_optimizers(self):
-            return [optim.SGD(self.parameters(), lr=1e-2), optim.SGD(self.parameters(), lr=1e-2)]
-
-    model = OldSignatureModel()
-    trainer = Trainer(default_root_dir=tmpdir, fast_dev_run=2)
-
-    with pytest.deprecated_call(match="`training_step` .* `optimizer_idx` .* manual .* will be removed in v1.5"):
-        trainer.fit(model)
-
-
 def test_v1_5_0_model_checkpoint_period(tmpdir):
     with no_warning_call(DeprecationWarning):
         ModelCheckpoint(dirpath=tmpdir)
diff --git a/tests/trainer/optimization/test_manual_optimization.py b/tests/trainer/optimization/test_manual_optimization.py
index fa1d2cd381..9ce3358fbe 100644
--- a/tests/trainer/optimization/test_manual_optimization.py
+++ b/tests/trainer/optimization/test_manual_optimization.py
@@ -1098,3 +1098,17 @@ def test_multiple_optimizers_logging(precision, tmpdir):
 
     assert set(trainer.logged_metrics) == {"epoch", "loss_d", "loss_g"}
     assert set(trainer.progress_bar_metrics) == {"loss_d", "loss_g"}
+
+
+def test_manual_optimization_training_step_signature(tmpdir):
+    """Test that Lightning raises an exception if the training_step signature has an optimier_idx by mistake."""
+
+    class ConfusedAutomaticManualModel(ManualOptModel):
+        def training_step(self, batch, batch_idx, optimizer_idx):
+            return super().training_step(batch, batch_idx)
+
+    model = ConfusedAutomaticManualModel()
+    trainer = Trainer(default_root_dir=tmpdir, fast_dev_run=2)
+
+    with pytest.raises(ValueError, match="Your `LightningModule.training_step` signature contains an `optimizer_idx`"):
+        trainer.fit(model)