Seattle, which went 13-3 this season to earn a playoff home-field edge and a first-round bye, will host the winner of Sunday's Carolina-San Francisco game on January 19 in the National Conference final for a Super Bowl berth.
The Seahawks denied a late comeback bid by Saints quarterback Drew Brees and moved within one victory of their first trip to the Super Bowl since losing to Pittsburgh in 2006.
"It's only a first step but it's a hell of a first step," said Seahawks coach Pete Carroll.
New England was set to host Indianapolis in an American Conference match-up later Saturday with that winner to face either Denver or San Diego in next week's American Conference final for the other Super Bowl spot.
On a windy and rainy day, the Seahawks took support from their notoriously loud fans and improved to 16-1 at home in the past two seasons.
Lynch, who carried the ball 28 times, scored on a 15-yard run in the second quarter and Steven Hauschka kicked field goals of 26, 38 and 49 yards to give the Seahawks a 16-0 half-time lead.
That was the same score entering the fourth quarter, the Seahawks' NFL-best scoring defense leaving the Saints without a point through three quarters for the first time since 2002.
Khiry Robinson scored on a 1-yard run for the Saints and Mark Ingram ran for a 2-point conversion to halve the Seattle lead with 13:11 remaining.
The Seahawks answered with a late drive capped by Lynch's 31-yard touchdown run with 2:40 to play for a 23-8 lead.
Brees marched the Saints 80 yards in nine plays and flipped a 9-yard touchdown pass to Marques Colston with 26 seconds remaining and Colston recovered an onside kick to give New Orleans a chance to win the game.
But Colston attempted an illegal forward pass after a reception three plays later and that penalty resulted in the final seconds being taken off the clock.
Brees completed 24-of-43 passes for 309 yards while Colston had 11 catches for 144 yards.