The 72-year-old, who had flown himself to Boston just for Saturday's 1:10 p.m. game against the Royals, surprised the 35,152 in attendance after the top of the eighth inning and sung the song that's made him synonymous with Fenway Park.
"Sweet Caroline" may have never sounded sweeter.
The Sox stormed back in the bottom of the inning to take the eventual 4-3 win.
Since Diamond's 1969 hit began making regular appearances at Fenway Park in 2002, the song has been considered the Red Sox's song. But this week, with the city suffering from the Boston Marathon bombing tragedy and the Red Sox in Cleveland from Tuesday through Thursday, "Sweet Caroline" was anything but Boston's song.
It rang out at ballparks across the country. Yankees fans were singing the song at Yankee Stadium.
It finally returned to Boston on Saturday afternoon.
After Clay Buchholz finished pitching his final inning, Diamond, who appeared down the first-base line wearing a black blazer and a Red Sox hat, softly sang his song while encouraging the crowd to join along.
The Fenway Faithful immediately followed with a U-S-A chant.
Jason Mastrodonato is a reporter for MLB.com. Follow him on Twitter @jmastrodonato. This story was not subject to the approval of Major League Baseball or its clubs.