People walking past found the man unconscious in the street near 8th and Franklin at about 3 a.m. Sunday, Boise police said.
Paramedics took the man to a local hospital with serious, but not life-threatening injuries.
After arriving at the hospital, medical staff determined the bicyclist had been hit by a vehicle. Meanwhile, the driver of the vehicle in question returned to the scene about 20 minutes after the crash, where he admitted he had hit a cyclist while driving, police said.
The driver told police he was heading down 8th Street when the bicyclist entered the road from the alley between Washington and Franklin, without stopping.
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The bicyclist was not wearing a helmet, police said.
Detectives continue to investigate the crash. There is no sign that the driver was impaired, police said. But charges are pending against the driver for leaving the scene of an accident.
No names of those involved have been released.