The 2009 Lamar Hunt U.S. Open Cup Final was played on September 2, 2009, at Robert F. Kennedy Memorial Stadium in Washington, D.C. The match determined the winner of the 2009 Lamar Hunt U.S. Open Cup, a tournament open to amateur and professional soccer teams affiliated with the United States Soccer Federation. This was the 96th edition of the oldest competition in United States soccer. The match was won by Seattle Sounders FC, who defeated D.C. United 2–1. Clyde Simms scored D.C. United's only goal. Fredy Montero and Roger Levesque scored Seattle's two goals as the club became the second expansion team in Major League Soccer (MLS) history to win the tournament in their inaugural season.

D.C. United entered the tournament as the competition's defending champions. They had previously won the tournament in 1996 as well. Both Sounders FC and D.C. United had to play through two qualification rounds for MLS teams before entering the official tournament. Prior to the final, there was a public dispute between the owners of the two clubs regarding the selection of D.C. United to host it at their home field, RFK Stadium.

As the tournament champions, Sounders FC earned a berth in the preliminary round of the 2010–11 CONCACAF Champions League. The club also received a $100,000 cash prize, while D.C. United received $50,000 as the runner-up.

The U.S. Open Cup is an annual American soccer competition open to all United States Soccer Federation affiliated teams, from amateur adult club teams to the professional clubs of Major League Soccer (MLS). In 2009, Major League Soccer, which has teams that play in both the United States and Canada, was allowed to enter eight of its U.S.-based teams in the tournament. The top six MLS teams from the previous season qualified automatically, and the remaining two spots are determined by preliminary qualification matches. The eight MLS entries begin play in the third round of the tournament.

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