diff --git a/CHANGELOG.md b/CHANGELOG.md
index 79b253061ddb2..70d2abcd7a7af 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
+
+- Added more explicit exception message when trying to execute `trainer.test()` or `trainer.validate()` with `fast_dev_run=True` ([#6667](https://github.com/PyTorchLightning/pytorch-lightning/pull/6667))
+
+
 - Trigger warning when non-metric logged value with multi processes hasn't been reduced ([#6417](https://github.com/PyTorchLightning/pytorch-lightning/pull/6417))
 
 
diff --git a/pytorch_lightning/trainer/trainer.py b/pytorch_lightning/trainer/trainer.py
index f0f1d3e6b11e1..fa02df7fb7ad1 100644
--- a/pytorch_lightning/trainer/trainer.py
+++ b/pytorch_lightning/trainer/trainer.py
@@ -955,31 +955,38 @@ def __load_ckpt_weights(
         model,
         ckpt_path: Optional[str] = None,
     ) -> Optional[str]:
-        # if user requests the best checkpoint but we don't have it, error
-        if ckpt_path == 'best' and not self.checkpoint_callback.best_model_path:
+        if ckpt_path is None:
+            return
+
+        fn = self.state.value
+
+        if ckpt_path == 'best':
+            # if user requests the best checkpoint but we don't have it, error
+            if not self.checkpoint_callback.best_model_path:
+                if self.fast_dev_run:
+                    raise MisconfigurationException(
+                        f'You cannot execute `.{fn}()` with `fast_dev_run=True` unless you do'
+                        f' `.{fn}(ckpt_path=PATH)` as no checkpoint path was generated during fitting.'
+                    )
+                raise MisconfigurationException(
+                    f'`.{fn}(ckpt_path="best")` is set but `ModelCheckpoint` is not configured to save the best model.'
+                )
+            # load best weights
+            ckpt_path = self.checkpoint_callback.best_model_path
+
+        if not ckpt_path:
             raise MisconfigurationException(
-                'ckpt_path is "best", but `ModelCheckpoint` is not configured to save the best model.'
+                f'`.{fn}()` found no path for the best weights: "{ckpt_path}". Please'
+                f' specify a path for a checkpoint `.{fn}(ckpt_path=PATH)`'
             )
 
-        # load best weights
-        if ckpt_path is not None:
-            # ckpt_path is 'best' so load the best model
-            if ckpt_path == 'best':
-                ckpt_path = self.checkpoint_callback.best_model_path
-
-            if not ckpt_path:
-                fn = self.state.value
-                raise MisconfigurationException(
-                    f'`.{fn}()` found no path for the best weights: "{ckpt_path}". Please'
-                    ' specify a path for a checkpoint `.{fn}(ckpt_path=PATH)`'
-                )
+        # only one process running at this point for TPUs, as spawn isn't triggered yet
+        if self._device_type != DeviceType.TPU:
+            self.training_type_plugin.barrier()
 
-            # only one process running at this point for TPUs, as spawn isn't triggered yet
-            if not self._device_type == DeviceType.TPU:
-                self.training_type_plugin.barrier()
+        ckpt = pl_load(ckpt_path, map_location=lambda storage, loc: storage)
+        model.load_state_dict(ckpt['state_dict'])
 
-            ckpt = pl_load(ckpt_path, map_location=lambda storage, loc: storage)
-            model.load_state_dict(ckpt['state_dict'])
         return ckpt_path
 
     def predict(
diff --git a/tests/trainer/test_trainer.py b/tests/trainer/test_trainer.py
index 4ca2f737f5106..ee93ca59eca76 100644
--- a/tests/trainer/test_trainer.py
+++ b/tests/trainer/test_trainer.py
@@ -1777,3 +1777,12 @@ def on_fit_start(self, trainer, pl_module: LightningModule) -> None:
 
     trainer = Trainer(default_root_dir=tmpdir, max_epochs=1, callbacks=[TestCallback()])
     trainer.fit(model, datamodule=dm)
+
+
+def test_exception_when_testing_or_validating_with_fast_dev_run(tmpdir):
+    trainer = Trainer(default_root_dir=tmpdir, fast_dev_run=True)
+
+    with pytest.raises(MisconfigurationException, match=r"\.validate\(\)` with `fast_dev_run=True"):
+        trainer.validate()
+    with pytest.raises(MisconfigurationException, match=r"\.test\(\)` with `fast_dev_run=True"):
+        trainer.test()