The accident occurred at about 1:30 p.m. Friday in the area of 355 Riverside Ave, police said.
Police, EMS and the Fire Department all arrived on the scene and found the motorcycle operator wedged underneath the rear of a box-type truck, police said.
EMS began medical treatment for the motorcyclist, who was taken to Norwalk Hospital with serious injuries, police said. The extent of the motorcyclist's injuries was still being determined.
The truck driver remained at the scene and was not injured, police said.
The crash remains under investigation by the Westport Police Department Accident Investigation Unit and Patrol Division.
Riverside Avenue was briefly closed after the accident but has reopened.
Click here to follow Daily Voice Westport and receive free news updates.