Cal Fire received a report of a major-injury accident at 3:04 p.m. Saturday on San Antonio Valley Road, several miles east of the Lick Observatory.
Sheriff's deputies responded and found a man unconscious and not breathing. He was pronounced dead within half an hour, Sgt. Rick Sung said.
The cause of his death won't be determined until toxicology test results come back, but there were no signs of foul play, Sung said.
The man, approximately 60 years old, was participating in the 41st Annual Mt. Hamilton Challenge and Ascent Bicycle Tours, "designed for experienced, self-reliant cyclists," according to the website.
The San Jose Fire Department also assisted.