The result handed idle Oregon the Pac-10 title and a Bowl Championship Series berth. The Ducks are off until hosting Oregon State on Dec. 1.
Washington (8-2, 6-2 Pac-10) can gain a share of the conference title if Oregon loses to Oregon State. But the Ducks hold the tiebreaker based on their 24-17 victory over the Huskies on Oct. 27.
Washington State was its own worst enemy as the 10th-ranked Cougars (9-2, 6-2) came up empty on three first-and-goal opportunities inside the six. The struggles inside the red zone included a failed fourth-down conversion, a fumble and an interception.
The Cougars were denied their first 10-win season since 1997, when Ryan Leaf led them to the Rose Bowl.
Reggie Williams caught 11 passes for 203 yards for Washington, which plays at No. 2 Miami next week.
After Washington took a 7-0 lead on Rich Alexis' one-yard run with 33 seconds left in the first quarter, the Cougars tied it on Jason Gesser's 15-yard pass to Mike Bush five minutes into the second period.
But Washington State's problems began after John Anderson booted a 29-yard field goal to give the Huskies a 10-7 lead with nine seconds left in the first half.
Jason David fumbled on the ensuing kickoff and Washington took advantage, grabbing a 13-7 halftime lead on Anderson's 35-yard field goal as time expired.
Washington State got the first possession of the second half, but again coughed up the ball when David Minnich fumbled inside the 10. The Huskies took a 19-7 lead on Willie Hurst's one-yard plunge just 61 seconds into the third quarter.
Washington State finally converted from the red zone on Gesser's five-yard pass to Nakoa McElrath with 3:58 remaining in the contest, but Washington recovered the ensuing onside kick and ran out the clock by picking up three first downs.