"This is amazing," one Red Sox fan said from the stands as Diamond walked onto the field. "It's like Christmas."
Diamond walked around the infield grass as he performed, while players from both the American and National League watched from the top step of their dugouts.
Fenway Park has played "Sweet Caroline" in the middle of the eighth inning during every Red Sox game since 2002. The song was played in stadiums around the country after the Boston Marathon bombings as a show of solidarity.
Tuesday was the first time the song had been performed live at Citi Field. Diamond's new single "Freedom Song" (They'll Never Take Us Down) was also played in the stadium during the game. All of the proceeds from downloads of the song will benefit Wounded Warrior Project and One Fund Boston.
Josh Vitale is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.