The crash happened at roughly 7 p.m. near the intersection of Highway 303 and Northeast Fairgrounds Road, north of Bremerton.
Deputy Scott Wilson with the Kitsap County Sheriff's Office said it appears a man driving a Pontiac Grand Am was heading east on Fairgrounds Road when he hit the fire truck.
"From what witnesses describe, the vehicle traveling eastbound started to cross the center line," Wilson said. "The fireman operating the fire truck realized this was a potential head-on collision, so they swerved and the truck veered off the road and went slightly into the ditch. Unfortunately, they couldn't avoid striking the vehicle all together."
Wilson said the Pontiac slammed into the side of the fire truck before coming to a stop in the middle of the road. The car was mangled in the crash, and the driver was briefly trapped inside.
After they freed him from the wreckage, medics rushed the driver to Harrison Medical Center in Bremerton. His injuries are described as being "serious," according to Wilson.
There were no other injuries. Wilson said the firefighter was not on an emergency call at the time of the crash.
It's unclear if drugs or alcohol played a role in the crash. The investigation is being handled by the Washington State Patrol.