Kent Fire Department spokesman Randy Droppert said the victim's body was found inside the Sunset Motel just after 7 a.m.
"Lady walking down the street stopped and said she smelled smoke," said hotel manager Charles Blackorby. He grabbed the keys to the rooms and ran to make sure everyone was out.
"I could see fire around the curtains and I could see fire around the bed and everything else was so smoky I couldn't go in so then I called 911," he said.
The one unit was destroyed and four others were heavily damaged by the fire, but no one else was injured.
Droppert said it was not clear how the person died or what started the fire.
He said the room where the body was found was rented by someone who had lived there for a long time, but the body has not been identified.
"There's a lot of work to be done yet," he said.
Investigators expect to be on scene at the motel at 25006 Pacific Highway South for much of the day.