The American, a student in the college of liberal arts and science at Arizona State University, is understood to have been in Phuket as an exchange student.
He and three fellow-students, two men and a woman, went swimming in front of the Gracelands Hotel.
Rescue workers said there were particularly strong rip currents last night, and all four were soon in trouble. Three were rescued, but the fourth - as yet unnamed - could not be found.
A search for him began at 8 this morning and continues.
Coincidentally the Marine Police were conducting a rescue drill off Patong when the disappearance was reported. The drill has now turned into the real thing.