Walker, signed this week after all-pro receiver Sidney Rice had hip surgery, scored from 25 yards out on fourth down and 4 yards to go with 7 minutes left in the game to wipe out a 13-10 Seattle lead.
Fourth-string quarterback Joe Webb then hit Ryan D'Imperio for a 7-yard TD after Charlie Whitehurst was intercepted.
Favre, who showed up late to training camp and played in just one series in the Vikings' last outing, completed 16-of-26 passes for 187 yards. One of his interceptions was returned 86 yards for a TD by Earl Thomas.
Adrian Peterson ran for 37 yards with 24 of them coming on a scoring jaunt.
The Vikings also got 47 yards on four catches from another new receiver, Greg Camarillo.
Seahawks quarterback Matt Hasselbeck hit 9-of-17 for 126 yards but Seattle could only must a pair of field goals by Olindo Mare.