The search was sparked by the disappearance of American student Joshua Shane, 21, who went missing last night at about 10pm when he went swimming with a group of friends off Patong Beach.
The American, a student in the college of liberal arts and science at Arizona State University, is understood to have been in Phuket as a short-term exchange student, working with Phuket City Municipality. He arrived in Phuket on Sunday.
He and three fellow-students, two men and a woman, went swimming in front of the Graceland Hotel.
Rescue workers said there were particularly strong rip currents last night, and all four were soon in trouble. Three were rescued, but Mr Shane could not be found.
Coincidentally the Marine Police were conducting a search and rescue drill off Patong when the disappearance was reported. They promptly turned their exercise into the real thing.
Mr Shane is 167 centimetres tall, with brown eyes and hair, and was wearing a white bathing suit when he disappeared. Anyone seeing him is asked to contact Chuthatip Maneepong at 087 028 6468.
The Marine Police will resume their search tomorrow.