The collision happened near the intersection of Occidental Ave. S. and S. Lander Street, Seattle Fire officials said.
Traffic was stopped on Lander for a train crossing and some cars began making a U-turn rather than waiting for the train to finish crossing, Seattle police said. However, one driver in an SUV turned into the motorcycle officer.
A quick-thinking witness in the car behind that driver rushed into action and helped the injured cop.
"I got on his radio which was clipped to his chest and let SPD know that they had an officer down," said Michael Katz. "He was still conscious and they let me know that they were coming."
The 57-year-old officer was taken to Harborview Medical Center where he is listed in serious but stable condition with broken bones, officials said.
Seattle police says expect traffic delays around the scene of the crash.