The champions were the first to register when Steven Hauschka kicked a 24-yard field goal, but they found themselves behind before the quarter was out as Tre Mason burst through the middle for a six-yard touchdown.
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The Rams stretched the gap to 18 points midway through the quarter as they faked a punt going to one side of the field, allowing Stedman Bailey to collect it on the opposite side and race 90 yards down the right touchline for an unorthodox score.
Hauschka reduced the gap to 15 with a 35-yard kick shortly before half time, while Seattle had their first touchdown of the evening in the third quarter as Russell Wilson raced in for a 19-yard score of his own.
The comeback continued as Wilson found Cooper Helfet for a 25-yard score, although they missed the chance to level things up when the attempted two-point conversion was unsuccessful.
That proved to be costly as the Rams moved back to more than a score ahead when Davis fired a pass to Lance Kendricks, who just about kept control of the ball long enough to reach into the endzone.
The Seahawks would not give in, however, and moved back to within two in the closing minutes when Doug Baldwin collected a nine-yard Wilson pass for a touchdown.
A fake punt allowed the Rams to make a first down in the closing stages, however, and they clung on to their lead to claim an unexpected victory.
St Louis now have a 2-4 record for the season, while the defending champions have lost back-to-back games for the first time since 2012 and sit with a 3-3 record.