April Wagner, 54, was riding on the back of a 2004 Harley Davidson motorcycle driven by Daniel McGillicuddy, 60, of Santa Rosa, when the crash happened around 2:45 p.m.
A witness said the motorcycle was heading north on E Street when it ran a red light and crossed Bennett Valley Road, colliding with a 2012 Kia, Santa Rosa police Sgt. Rich Celli said.
Both riders were found lying in the road when police arrived, Celli said.
Wagner and McGillicuddy each suffered a broken leg in the collision. Wagner also suffered a broken pelvis and head injury, Celli said.
She was in critical condition and McGillicuddy was in fair condition this morning, hospital spokeswoman Katy Hillenmeyer said.
The driver of the Kia, Anne Foster, 33, of Santa Rosa, was not injured, Celli said.