The 1909 Pittsburgh Pirates season was the 28th season for the Pittsburgh Pirates franchise, during which they won the National League pennant with a record of 110–42 and their first World Series over the Detroit Tigers. Led by shortstop Honus Wagner and outfielder-manager Fred Clarke, the Pirates scored the most runs in the majors. Wagner led the league in batting average, on-base percentage, slugging percentage, and runs batted in. Pirates owner Barney Dreyfuss opened the Pirates' new ballpark, named Forbes Field, on June 30, 1909.

The 1909 Detroit Tigers won the American League pennant with a record of 98–54, but lost to the Pittsburgh Pirates in the 1909 World Series, 4 games to 3. The season was their ninth since they were charter members of the American League in 1901. It was the third consecutive season in which they won the pennant but lost the World Series. Center fielder Ty Cobb won the Triple Crown and pitcher George Mullin led the league in wins (29) and winning percentage (.784).