The announcement was made Friday by Stu Jackson, the league's executive vice president for basketball operations.
Garnett was fined for comments he made following the Celtics' 109-104 loss to the Oklahoma City Thunder on Wednesday.
Thunder forward Kevin Durant made all 15 of his free throws against Boston, prompting Garnett to say, "I thought we were playing Michael [expletive] Jordan the way he was getting the whistle. Durant damn near shot more free throws than our whole team."
Boston went 13-for-17 from the line.
Information from The Associated Press was used in this report.