Leo Cullen, 56, of Fairfield, suffered life-threatening injuries in the 5:10 p.m. collision with a 2009 BMW 328i, California Highway Patrol officials said.
At the time of the accident, Cullen was riding east in the bicycle lane on Rockville Road near Interstate Highway 80, CHP Officer Chris Parker said.
Teresa Booth, 47, of Vallejo, was driving her BMW west on West Texas Street, which becomes Rockville Road at the highway, and turned left onto the I-80 west onramp, Parker said.
Witnesses said Booth had the green left-turn arrow and Cullen rode through a red light, across the on-ramp and into the path of the BMW, Parker said.
Cullen was not wearing a helmet and was ejected from his bike. He was taken to John Muir Medical Center in Walnut Creek, where he remained in critical condition Thursday morning, a medical center spokeswoman said.