Coach Jeff Tedford made the announcement about his quarterbacks after practice on Thursday. The Golden Bears open the season Aug. 30 at home against the Spartans.
"We feel like both of them have had great camps, but Kevin's going to take the first snaps and we'll see how it goes from there," Tedford said. "Nate will play in the game. I don't know exactly when, but Kevin will start."
Riley saw action in four games last year as a redshirt freshman, throwing for 563 yards and five touchdowns.
In Riley's first career start last season, he tried to run with the ball in the final seconds instead of throwing it away, costing the Bears the chance at a game-tying field goal in a 31-28 loss to Oregon State. Cal would have been in position to move up to No. 1 in the poll with a win, but instead lost six of its final regular season games.
Riley did excel in the Armed Forces Bowl for Cal, entering with a 21-0 deficit in the second quarter and leading the Bears to a 42-36 victory over Air Force. Riley was 16-for-19 for 269 yards and three touchdowns to earn MVP honors in the game.
"I'm just getting ready for Michigan State," Riley said. "It's an honor to be named the starter, especially after what Nate's done at Cal. We're in good hands having two good quarterbacks here. I just have to prepare, watch film and get ready for this game."
Longshore ranks fourth all-time at Cal in passing efficiency (133.1), sixth in passing touchdowns (62) and seventh in passing yardage (5,732). He is also second all-time at Cal with 18 victories as a starting quarterback.