Boise police are investigating the crash, which happened just before 2:30 p.m. Thursday.
Witnesses told police a car driven by a man south on Parkcenter struck the bicyclist as he crossed at Pennsylvania.
The bicyclist was taken to a local hospital with life-threatening injuries, police said, but information on his condition was not available Thursday night. The driver was not reported as injured.
Southbound Parkcenter was closed to traffic for much of Thursday afternoon as a crash reconstruction team worked at the scene.