Sheriff's deputies say the truck, carrying a total of seven teens, went off the road and crashed into a tree along Forest Service Road 23 about 18 miles northwest of Shelton. Five teens in the back of the pickup and were ejected at high speed, the sheriff's office said.
The driver was airlifted to Harborview Medical Center in Seattle for treatment of a broken leg and facial cuts. Two others were taken to Mason General Hospital and the other two survivors were treated at the scene.
Detectives say they are investigating whether alcohol and/or marijuana were factors in the crash.
Those killed in the crash have been identified as 19-year-old David Michael Dudley, Jr. of Bremerton and 18-year-old Alena J. Pepi of Port Orchard.
"Our heartfelt condolences go out to the family and friends of not only those who were hurt - but those who perished in this," said Det. Willian Adam with the Mason County Sheriff's Office.