Sam Darnold has thrown for almost 300 yards and has three touchdown passes, the last one to Cooper Kupp.
The ex-Rams wide receiver scored on a 13-yard reception, giving the Seahawks a 31-20 lead with 4:52 remaining in the third quarter. Kupp spent his first eight seasons with the Rams before joining the Seahawks in the offseason after Los Angeles moved on.
He caught 47 passes for 593 yards and two touchdowns in the regular season.
The second half has turned into a shootout with the Seahawks scoring two touchdowns and the Rams one in the third quarter.
Darnold had a 14-yard touchdown throw to Jaxon Smith-Njigba late in the second quarter and had a 17-yarder to Jake Bobo in the third quarter after Xavier Smith's muffed punt.
Darnold is 19-of-27 for 291 yards.
Smith-Njigba has nine catches for 139 yards and a touchdown.