Beach guards raced out in an inflatable dinghy at about noon to help after Iliya Muryndin, 29, was seen crashing his rented jet-ski in high waves.
When they reached him they found him still breathing but unconscious from a blow he had received to his right forehead.
Back on shore, they rushed him to Patong Hospital. He briefly woke up but then lapsed again into unconsciousness. It was decided, because of his condition, that he should be transferred to Vachira Phuket Hospital, but he died on the way there.
Police are gathering information about the victim and investigating the cause of the incident. The family have been informed.