The man, Vladimir Slobodenyuk, 29, was seen earlier walking around the Nai Harn lagoon, then later found hanging from a tree near a football field.
He was dressed in a light-blue t-shirt and white check shorts. There were no signs of marks on his body, said officers from Chalong Police Station.
It's not known whether the man was an expat or a tourist. He had a patient's card from Bangkok Hospital Phuket, in Phuket City, in a pocket.
There is no clarity yet on what motive he might have for suicide.
Police believe he climbed a tree, attached a rope, and jumped. His body was taken to Vachira Phuket Hospital in Phuket City.