Officers on patrol saw the man in front of a 7-Eleven on the Karon beach road at about 10pm, Karon Police Chief Col Peerapong Chai-aroon told The Phuket News.
The patrol officers witnessed as the man raised the knife to his neck, and cut from one side of his neck to the other, Col Peerapong said.
The police officers rendered immediate assistance and an ambulance soon arrived to take the man to Patong Hospital.
The man was later taken to Vachira Phuket Hospital in Phuket Town, where he has undergone surgery and remains in the intensive-care unit (ICU), he added.
Police have yet to confirm the man’s identity, Col Peerapong said.
Meanwhile, officers from the Tourist Assistance Centre (TAC) told The Phuket News they believe the man is a Russian national.