The punishment follows comments made by Martinez after the match against Manchester City last season.
Martinez had been furious at referee Stuart Attwell for the second half dismissal of defender Gary Caldwell for a tackle on Carlos Tevez during the Latics' 3-0 defeat at Eastlands on March 29.
At a personal hearing on Wednesday, the charge was proven and, taking into account Martinez's previous exemplary record, the Independent Regulatory Commission warned him as to his future conduct and fined him £2,250.
Caldwell's red card for a challenge on Tevez that Attwell described as being with "excessive force" proved pivotal as the Argentina striker proceeded to score a 13-minute hat-trick.
Immediately after the game Martinez said: "We are all human beings and we all make mistakes, but to give a decision after something you don't see, that is lying. You are making it up.
"He (the referee) said he saw Gary Caldwell with both feet off the ground. That's why he gave the red card.
"But Gary Caldwell only went with one foot. That is a real injustice."'