The tourist arrived in Krabi Province from Phuket with a tour guide late last weekend and went snorkeling at Monkey Bay, The Phuket Express reported on Tuesday.
His body was later found floating in the sea and police suspect that he drowned.
An autopsy has been conducted to determine the cause of death, and his embassy has been notified.
In recent weeks, several foreign tourists have drowned in Thailand’s popular vacation hotspots.
Thailand received 435,000 Russian visitors last year, 14 times more than in 2021, thanks to the easing of entry policies that allow them a 45-day visa-free stay.