16 troops were aboard the AAV when it began taking on water.
One US Marine has been killed and eight more are missing after a training accident off the southern California Coast.
The incident occurred on Thursday evening aboard an AAV (amphibious assault vehicle) as the 15th Marine Expeditionary Unit and Makin Island Amphibious Ready Group carried out a routine near San Clemente Island.
16 servicemen were on board when the vehicle reported it was taking on water around at 5:45 p.m.
"We are deeply saddened by this tragic incident. I ask that you keep our Marines, Sailors, and their families in your prayers as we continue our search," Col. Christopher Bronzi, the 15th MEU's commanding officer, said in a statement.
AAVs are tank-like vehicles that can travel on land or water, used for transporting troops from ship to shore.
In 2011, another AAV out of Camp Pendleton sank during a training exercise with six Marines on board; five made it out alive, but one was tragically killed.