The 2009 World Series was the 105th edition of Major League Baseball's (MLB) championship series. The best-of-seven playoff was contested between the Philadelphia Phillies, champions of the National League (NL) and defending World Series champions, and the New York Yankees, champions of the American League (AL). The Yankees defeated the Phillies, four games to two, to win the franchise's 27th World Series championship. The series was played between October 28 and November 4, broadcast on Fox, and watched by an average of roughly 19 million viewers.

Home field advantage for the Series went to the AL for the eighth straight year as a result of its 4–3 win in the All-Star Game. The Phillies earned their berth into the playoffs by winning the National League East. The Yankees won the American League East to earn their berth, posting the best record in the Major Leagues. The Phillies reached the World Series by defeating the Colorado Rockies in the best-of-five National League Division Series, and the Los Angeles Dodgers in the best-of-seven Championship Series (NLCS). The Yankees defeated the Minnesota Twins in the American League Division Series and the Los Angeles Angels of Anaheim in the Championship Series (ALCS) to advance to their first World Series since 2003.

The World Series is the annual championship series of Major League Baseball (MLB), played between the American League and National League champions since 1903. The winner of the World Series championship is determined through a best-of-seven playoff and awarded the Commissioner's Trophy. As the series is played in October, Major League Baseball also refers to it as the "Fall Classic". The most recent World Series was won by the St. Louis Cardinals, who defeated the Texas Rangers in 7 games in 2011.

The New York Yankees of the American League have played in 40 World Series and won 27, and the Oakland/Philadelphia Athletics have played in 14 and won 9. In the National League, the St. Louis Cardinals have played in 18 and won 11, while the San Francisco/New York Giants and Los Angeles/Brooklyn Dodgers have both appeared in 18 and won 6.

Until the formation of the American Association in 1882 as a second major league, the National Association of Professional Base Ball Players (1871–75) and then the National League (founded 1876) represented the top level of organized baseball in the United States. All championships went to whoever had the best record at the end of the season, without a postseason series being played. Starting in 1884 and going through 1890, the National League and the American Association faced each other in a series of games at the end of the season to determine an overall champion. These matchups were disorganized in comparison to the modern Series: games played ranged from as few as three in 1884 (Providence defeated New York 3 games to zero), to a high of 15 in 1887 (Detroit beat St. Louis 10 games to 5), and both the 1885 and 1890 Series ended in ties, each team having won three games with one tie game.