Robbie Findley scored in the 29th minute on Sunday to give Real Salt Lake a 1-0 victory at Portland Timbers and a berth against Sporting Kansas City in the MLS Cup final.
Real Salt Lake had won 4-2 at home in the first leg of the Western Conference semi-final two weeks ago and then blanked the Timbers before a loud Portland crowd to advance 5-2 on aggregate.
Sporting beat Houston 2-1 on Saturday to clinch the Eastern Conference crown and ensure Kansas City would be the site of the MLS Cup, the season championship match, on 7 December.
In their only meeting this season, Sporting Kansas City edged host Real Salt Lake 2-1 on 20 July.
Real Salt Lake beat the Los Angeles Galaxy to capture the 2009 MLS Cup in their lone prior trip to the final, winning 5-4 on penalties after the clubs played to a 1-1 draw.
Kansas City's only prior MLS crown came in 2000 when the club was known as the Wizards and beat Chicago 1-0 in the championship game at Washington. In 2004, the Wizards lost 3-2 to DC United in the MLS Cup final.
The Timbers, in only their third MLS season, edged Real Salt Lake for the Western Conference regular-season title. It was Portland's first winning MLS campaign with 14 triumphs, 15 drawn and only five defeats.