The MH-60S helicopter, which was carrying six people, crashed around 60 nautical miles off San Diego at 4:30 p.m., the Navy said, NBC News reported.
One crew member was rescued, officials said Tuesday.
The Coast Guard and the Navy were conducting search and rescue operations for the five missing service members, the US 3rd Fleet said.
The fleet said the helicopter was "conducting routine flight operations" when it crashed. It was from the USS Abraham Lincoln, an aircraft carrier.