Bulgarian national Plamen Petkova, 32, entered the water at West Wittering, West Sussex, after the five-year-old was being carried away by the tide.
He managed to reach the girl and pass her to a woman who brought her back to her family on the shore at midday on Saturday, Sussex Police said.
But Mr Petkova, of Westmoreland Drive, Sutton, Surrey, failed to reach dry land himself and he was recovered unconscious from the water by others at the scene.
An inquest has been opened and adjourned, a police spokesman said.
The girl, who has not been named, was on the beach with her family who are from north west London. The woman who was handed the girl in the sea was with a group separate from the child and Mr Petkova.
The tragedy happened as the beach was packed with sun-seekers enjoying a mini-heatwave at the weekend, with temperatures in parts of southern England hitting 28C (82.4F).