The ex-boyfriend, identified as 19-year-old Jarod Lane, is considered armed and dangerous. Renton police are asking anyone with information about his whereabouts to contact them immediately.
The 17-year-old girl, who is a student at nearby Lindbergh High School, was found inside a home in the 12700 block of SE 164th Street after neighbors called 911 to report smoke coming from the second floor just before 8 a.m.
Deputy Chief Erik Wallgren said firefighters arrived to find a couch on the second floor burning and the girl on the floor, unconscious. She showed no vital signs and attempts to revive her were unsuccessful.
Wallgren said the girl was the only person in the house.
Police spokeswoman Terri Vickers said injuries were found on the victim's body, and police later said her death appears to be a homicide.
Vickers also said the fire appears to be arson.
Police are looking for a car that Lane was driving, a 2001 green Mazda Protege with Washington license plate 077-YRD. Lane is known to have been at a gas station in North Bend on Friday morning, and is believed to be heading east.
Lane is described as a white male with a dark complexion, 5-feet-9-inches tall, weighing 152 pounds, with brown eyes and brown or black hair.