The NBA confirmed on Thursday that Arenas had been fined but did not say what for or how much the penalty was.
Arenas later revealed on the social media network that he was fined for an offensive joke he had made.
He apologized for his comments and asked the NBA for a copy of their rules for posting on social networking sites.
Arenas, 29, is a three-time NBA All-Star but has fallen foul of officials before.
In the 2009-10 season, when he was playing for the Washington Wizards, he was suspended for storing unloaded handguns in the team locker room.
He was traded to Orlando late last year.
(Reporting by Gene Cherry in Raleigh; Editing by Julian Linden)