Tight end Eric Saubert executed the game-winning two-point conversion for Seattle.
Seattle engineered a stunning fightback to overcome the Rams in extra time, clinching their place in the postseason.
Overcoming a 30-14 hole in the final period, the Seahawks triumphed 38-37 at home, notching their fifth consecutive victory.
While the Rams had become the first to earn a playoff berth earlier on game day, the Seattle team booked their return to the play-offs after a two-year absence.
With this result, the Seahawks now lead their division's standings with a twelve and three record, replacing the Rams, who dropped to the fifth seed with an eleven and four standing.
Quarterback Sam Darnold had two interceptions but recovered to deliver two touchdown passes, including the second score to star receiver Jaxon Smith-Njigba in overtime.
The visitors held a 13-7 lead at half-time before Kenneth Walker III broke free for a 55-yard scoring dash for the home side.
Rams cornerback Wallace then returned Darnold's initial interception for 56 yards, setting up a short score by Blake Corum on the ensuing snap.
In the final stanza, a touchdown from Matthew Stafford to Nacua pushed the Rams' lead to 30-14, but Rashid Shaheed ignited the hope with a spectacular return for a score.
Darnold then connected with AJ Barner in the endzone, and a successful conversion forced the contest into overtime.
Although Nacua scored for his second TD in OT, Smith-Njigba responded in kind to setup the game-winning conversion.
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