Emergency personnel responded to the scene, in the 28600 block of Military Road South, at about 10:50 a.m. after receiving 911 calls, said Gordon Olson, assistant chief of South King Fire & Rescue.
The victim, a man believed to be in his 40s, was conscious when medics first arrived, but he later lost consciousness. He had a possible head injury and broken bones. It appeared he was not wearing a helmet.
The driver of the car, a 31-year-old man, was taken into custody to be evaluated for contributing factors such as drugs or alcohol, said Cathy Schrock of the Federal Way police.
Witnesses at the scene told officers the car was swerving recklessly before it hit the bicyclist, Schrock said.
The roadway has been cleared, and is now open for drivers.