The 57-year-old man sustained a deep wound to his left leg last Friday while he was swimming in the water, Nation Thailand reported.
Lifeguards from a nearby hotel reported that the tourist had signaled for help. Upon reaching the tourist, they discovered the severe bite wound.
The tourist was then transported to a hospital in Phuket Province for treatment.
Although initial speculation pointed to a shark attack, authorities are awaiting expert analysis to confirm the species of fish involved, Bangkok Post reported.
Authorities confirmed that the tourist's condition is now stable.
Following the incident, authorities have put a warning sign at Khao Lak Beach to alert visitors to the potential danger.