The O'Brien Trophy, or O'Brien Cup, as labelled on the trophy itself, is a retired trophy that was awarded in the National Hockey Association (NHA) and the National Hockey League (NHL) ice hockey leagues of North America from 1910 to 1950. It was originally donated to the NHA by Canadian Senator M.J. O'Brien in honor of his son, Ambrose O'Brien. The Cup was fabricated using silver from an O'Brien mine.
The Cup has been awarded under four definitions. From 1910 through 1917, it was awarded to the NHA champion. In 1921, the Cup was transferred to the NHL and awarded to the NHL champion until 1927. From 1928 until 1938, it was awarded to the Canadian Division champion. Starting with the 1938–39 season, it was awarded to the NHL playoff runner-up. In 1950, the Cup was retired and has not been awarded since. In total, the Cup has been awarded in 41 seasons to twelve different teams. The Cup is now in the collection of the Hockey Hall of Fame.