Boston Red Sox fan Damon and chat show host Kimmel, who supports the Los Angeles Dodgers, wore matching red and blue 'I'm With Stupid' T-shirts, with arrows pointing at each other, as they cheered on their respective teams.
Ben Affleck joined them at the big game, which the Red Sox won to clinch the World Series title.
Kimmel and Damon's feud has been running for years on the comedian's late-night "Jimmy Kimmel Live!" show, with the "Good Will Hunting" often a figure of fun.
The host always signs off the show by apologising for running out of time to interview Matt, who once kidnapped Kimmel and took over his programme.