A lifeboat crew pulled a man from the sea off Brighton pier after he plunged into water just one metre deep in what is feared to have been an attempt at "tombstoning".
The man, in his late twenties and believed to be from London, was landed on the beach by the lifeboat and taken to hospital by ambulance.
The incident happened early on Saturday evening, at low tide.
Tombstoning is the practice of jumping from the tops of cliffs and other high points into the sea.
An RNLI spokesman said: "Where the casualty jumped he fell over 12 metres into no more than one metre of water."