Roberts' most significant moment in his career came in the 2004 postseason when he pinch ran for Kevin Millar after drawing a walk from Yankees closer Mariano Rivera. Rivera threw to first base three times (the last almost picked off Roberts); on the next pitch, Roberts stole second base. Bill Mueller followed with a single. Roberts scored to tie the game. The Sox went on to win in twelve innings, and eventually, the rest of their games in the postseason to win the ALCS and the World Series.
Roberts' ninth-inning steal is often referred to as "The Steal of the Century", "The Steal Heard Round the World", or simply "The Steal."
Roberts' ninth-inning steal in Game 4 of the 2004 ALCS earned him a place in Red Sox history. In 2006, the event was recognized as a Memorable Moment in Red Sox history by the Boston Red Sox Hall of Fame.
The following night, in Game 5, Roberts ran for Millar again in the eighth inning after Millar walked. The Red Sox were again trailing 4-3 and Tom Gordon was on the mound for the Yankees. Although Roberts never stole second, his presence on first base split Gordon's attention; Gordon seemed to focus more on keeping Roberts from stealing than on pitching to Trot Nixon at the plate. Nixon singled to center and Roberts advanced to third base; he then scored the tying run on a Jason Varitek sacrifice fly. The Red Sox won the game in the 14th inning on a David Ortiz single.
On June 15, 2007, Roberts, as a San Francisco Giant, returned to Fenway Park for the first time since 2004. He received a standing ovation every time he came up to bat.