Police were called to the scene, at the southern end of the beach at 11:40am.
The victim, 48-year-old Nikita Tretiakov, was found unconscious in the sea approximately 100 metres from shore, after disappearing while swimming with his son.
Lifeguards and emergency responders from Karon Municipality brought him to shore and performed CPR, but were unable to revive him. He was pronounced dead at the scene.
According to police, Mr Tretiakov had entered the sea with his son after leaving their hotel, about 10am. His wife remained on the beach.
About 30 minutes later, he was seen disappearing beneath the waves. Lifeguards were alerted and launched a search, locating his body around 11am.
Despite efforts from lifeguards and staff from Chalong Hospital, Mr Tretiakov could not be revived. His wife and son requested that he be transported to the hospital for further resuscitation attempts, but officials confirmed he had died at the scene.
Authorities have urged beachgoers to obey red flag warnings, which are placed to signal rough surf and dangerous swimming conditions, especially during the southwest monsoon season.
The warning to obey red flags at the beaches followed three tourists drowning at Phuket beach in just two days.