Dani Rivas, 27 from Galicia and Bernat Martínez, 35, from Valencia were taking part in the World Superbike race on Sunday when they were both involved in a first lap crash.
"A chain-reaction, multi-bike incident on the first lap of today's second MotoAmerica Superbike/Superstock 1000 race at Mazda Raceway Laguna Seca has resulted in the deaths of two competitors," MotoAmerica, which was supporting the event, said in a statement.
Martínez was taken from the raceway by ambulance to Community Hospital of the Monterey Peninsula (CHOMP) in Monterey, where he died from his injuries, while Rivas was transported by air ambulance to Natividad Medical Centre in Salinas where he also succumbed to his injuries.
"I really don't have any words to describe what we all feel," said MotoAmerica president Wayne Rainey.
"First and foremost, is the loss to the families and loved ones. Our sincere condolences goes out to each and every one of them as well as the MotoAmerica family. They are in our thoughts and prayers."
"We are deeply saddened to learn the news of the passing of Bernat Martínez and Daniel Rivas," said Gill Campbell, CEO and general manager of Mazda Raceway Laguna Seca. "Our prayers and hearts go out to their families and teams."