Landon Donovan's pro soccer team straight-up walked off the field in protest Wednesday ... after a member of his squad claimed an opponent used a homophobic slur on the pitch.

Donovan's San Diego Loyal team was playing against Phoenix Rising FC in a USL game in San Diego ... when one of Landon's players said a verbal insult was hurled his way just before halftime.

The player, Collin Martin -- an openly gay man -- reportedly claimed Junior Flemmings called him a "batty boy" ... which is known in Jamaican dialect as an anti-gay slur.

Donovan, Martin and the whole Loyal team were livid over the alleged comment ... and demanded referees or Phoneix Rising FC's coach remove Flemmings from the game.