RNLI and coastguard rescue crews pulled the man from the water in Charlestown near St Austell in Cornwall at about midnight on Saturday, said police.
He was taken by ambulance to the Royal Cornwall Hospital in Truro where he was pronounced dead.
Devon and Cornwall Police said the 25-year-old man's death was being treated as a "tragic accident".
His name has not yet been released but next of kin have been informed.
A man has died after falling from a quay where it is thought he had been fishing.