The accident was reported shortly before 4 p.m. near the intersection of Columbus Avenue and Montgomery Street, San Francisco fire spokeswoman Lt. Mindy Talmadge said.
The 40-year-old man was riding his bike against the one-way traffic on nearby Washington Street prior to the fall, police said.
For an unknown reason, the man lost control of his bike, fell and hit his head on the ground, Talmadge said.
He was taken to San Francisco General Hospital with life-threatening injuries, police said. An update on his condition today was not immediately available.