It happened around 2:00 p.m. on Route 40 near Route 7 in front of Governor's Square Shopping Center.
Police say the 28-year-old bicyclist suffered serious head trauma and a broken leg.
He was taken to Christiana Hospital in serious condition.
The pickup truck could be seen with damage to its front end and windshield.
The driver, a 73-year-old man, remained at the scene.
There is no word yet if he will be charged.