In 2004, Ramirez led the AL in home runs (43), slugging percentage (.613) and OPS (1.009); he finished second in errors committed as a left fielder (7), third in RBIs (130), fourth in doubles (44) and total bases (348), sixth in on-base percentage (.397), eighth in walks (82), tenth in runs (108), and posted a .308 batting average. He also led the AL in salary, at $22.5 million.
In the 2004 Major League Baseball All-Star Game, Ramirez hit a two-run home run off Roger Clemens in the top of the first inning, giving his teammates a 3–0 lead. Ramirez, Derek Jeter (with a single), Ichiro Suzuki (with a double) and Iván Rodríguez (with a triple) became the first All-Star quartet that combined to hit for the cycle during the same inning. The Red Sox again qualified for the postseason, and Ramirez continued his hot hitting. He hit .385 in the 2004 ALDS, leading the Red Sox to a sweep over the Anaheim Angels, and setting up a rematch in the 2004 American League Championship Series against the Yankees. Despite not driving in a run in the ALCS, Ramirez hit .300 as the Red Sox completed a historic comeback from down 3 games to 0 to win in seven against the Yankees, setting up a showdown with the St. Louis Cardinals in the 2004 World Series as the Red Sox attempted to end the Curse of the Bambino and win their first championship since 1918. Ramirez proceeded to hit .412 with a home run and 4 RBI in a 4-game sweep of the Cardinals, as the Red Sox won their first title in 86 years. His home run came in the first inning of Game 3, off Cardinals starter Jeff Suppan, continuing a streak throughout the Series where the Red Sox scored in the first inning of each game. Ramirez's 2004 season was capped off by being named World Series MVP. Ramirez also finished third in AL MVP voting, just ahead of teammate Ortiz, who finished fourth.
On May 10, 2004, Ramirez missed a Red Sox game to become a naturalized citizen of the United States. He entered the next game running onto the field to a standing ovation while carrying a small American flag held in his hand. He planted the flag in the left outfield corner of the field, in the shadow of the Green Monster, where it remained for the entire game.