After the 2002 season, the Marlins sold Kevin Millar to the Chunichi Dragons of the Japanese Central League. In order for the transaction to be completed, he first had to clear the waivers requested by the Marlins, but the Red Sox broke an unwritten rule and blocked the deal with a waiver claim. Millar had signed a two-year, $6.2 million contract with the Dragons in January 2003, but in an unprecedented deal brokered by MLB, the Marlins later repaid the money that the Dragons had paid for Millar, and the Red Sox paid a similar sum to the Marlins in return for Millar. On February 15, 2003, Millar was officially traded to the Red Sox. His clubhouse presence and offensive production helped spark the Red Sox to the 2003 American League Championship Series and the 2004 World Series.

During the 2003 playoffs, Millar began using the phrase "Cowboy Up", and in 2004 referred to his team as "idiots" to keep teammates loose during the stretch run to the World Series Championship.

On April 20, 2012, Millar, together with Pedro Martínez, gave a toast to Fenway Park on the 100th anniversary of the ballpark. Millar and Martinez stood on top of the home dugout and gave a toast that was the largest in history, according to the Guinness Book of World Records.

This hand-signed photo, framed photo of Kevin Millar will be a great addition to any sports or Red Sox collection.