The accident happened as the 23-year-old woman was riding her bike south on Bellflower Drive shortly before 7 p.m.
Witnesses told officers the woman was traveling at a high speed when she entered the intersection of Bellflower Drive and Hillcrest Avenue and hit the center median, police said.
She was thrown from the bike, and although there was traffic heading in both directions, she was not hit by a vehicle.
The woman was not wearing a helmet and appeared to have a serious head injury, police said.
She was airlifted to a hospital and is in critical condition.