Whitehurst, backing up injured starter Matt Hasselbeck, made his second career start at quarterback and responded by going 22-of-36 through the air for 192 yards, including a 4-yard scoring strike to Williams in the first quarter.
Olindo Mare booted three field goals and the Seahawks (7-9) became the NFL's first-ever sub-.500 division winner.
They broke a three-game losing streak and advanced to the playoffs instead of the Rams (7-9), who finished with an identical record but lost the tie-breaker on divisional play records.
Marshawn Lynch gained 75 yards on 20 carries and Ruvell Martin grabbed three passes for 85 yards for Seattle.
Sam Bradford was held to 19-of-36 passing, gaining 155 yards and yielding an interception for St. Louis, which was limited to a paltry 184 yards of total offense.