According to officials, the man was last seen shortly before 3pm Saturday near 17th St. and Seawall in Galveston.
Witnesses told authorities he was swimming with friends when he went under and never resurfaced.
USCG began its search for the missing swimmer immediately, bringing a rescue helicopter and response boats to the scene.
The Galveston Fire Department and Galveston Beach Patrol also assisted.
Officials resumed their search Sunday morning, combing more than 32 square miles for more than 10 and a half hours to no avail.