"They played better. They won and we lost. I'm proud of the football team for what they accomplished. ... The disappointment is all we feel right now," St. Louis coach Steve Spagnuolo told reporters.
The Rams finish 7-9 just one season after going a league-worst 1-15 a year ago and after winning an NFL-low six games in the last three seasons. St. Louis was attempting to become just the third 8-8 division champion in NFL history, joining the 1985 Cleveland Browns and 2008 San Diego Chargers.
Instead, Seattle (7-9) becomes the first sub-.500 team to make the playoffs.
The St. Louis offense was kept out of the end zone for the second time this season as the Rams had to settle for field goals by Josh Brown in the second and third quarters.
Rookie quarterback Sam Bradford completed 19 of 36 passes for 155 yards and one interception, which came in the fourth quarter as he tried to rally the Rams for a late comeback attempt.
Running back Steven Jackson rushed for 45 yards on 11 carries and had four receptions for another 39 yards.