Red Sox Pitcher Ryan Dempster -- DECLARES UGLY SUIT WAR

Red Sox pitcher Ryan Dempster is a competitor -- so he wasn't gonna just sit back and let L.A. Dodgers star Andre Ethier get away with rocking the ugliest suit during the MLB playoffs .. and today, DEMPSTER OUT-UGLIED ETHIER!

The 36-year-old (2-time All-Star) threw on his loudest orange yellow and brown plaid suit to board the team jet for the trip to St. Louis for Game 3 of the World Series.

Of course, Ethier broke out his finest ugly threads when the Dodgers flew to St. Louis for the NLCS earlier this month ... and they lost.

Ugly suit jinx? Guess we'll have to wait and see.