Orton finished with two touchdowns among his 25 completions on 35 attempts. He found Thomas with six passes for 97 yards, including a 21-yard scoring strike.
Thomas was making his Broncos debut after sitting out Denver's first game and all of the pre-season with a foot injury.
Knowshon Moreno and Correll Buckhalter both scored rushing touchdowns and Eddie Royal hauled down a 13-yard touchdown among his five receptions for Denver (1-1).
Seattle signal-caller Matt Hasselbeck (20-of-35, 233 yards) threw three interceptions. Two of them came inside the Denver 10-yard line.
Seattle (1-1) turned the ball over four times. A key miscue came when rookie Walter Thurmond muffed a punt at his own 13-yard line in the first quarter and Denver quickly capitalized for seven points.