McCain, the Republican frontrunner, already leads the polls over Mitt Romney, his closest rival and a former Massachussets governor, in California.
The popular governor of California made the announcement in California alongside McCain, the senator for Arizona, and Rudy Giuliani, the former New York mayor who dropped out of the race on Wednesday and is also backing McCain.
"There are people out there that talk about reaching across the aisle, but he has shown the action, over and over again," Schwarzenegger said.
McCain has been backed by a large number of independent voters, but his strength among traditional conservative Republicans remains under question.
He said on Wednesday he believes he can unite with conservatives over what he says is the threat to the US posed by what he calls Islamic extremism.