It happened around 10:00 a.m. Thursday on West Chester Pike near Interstate 476 in Broomall.
The victim, a 38-year-old man, was riding westbound when he was hit from behind by a woman driving a Honda Accord.
The bicyclist was launched from the bike and suffered severe head injuries.
He was flown to the Hospital of the University of Pennsylvania in critical condition.
The driver of the Honda stayed at the scene. Police could not yet say what distracted her.