The league announced the $25,000 penalty on Monday.
Barnes was thrown out with 56.8 seconds left in the period after he was whistled for a flagrant 2 foul against Kevin Love on a drive to the basket. The Clippers went on to a 120-116 overtime victory.
Late Monday night, Barnes reacted with dismay on Twitter to the league fine.
"I appreciate the @NBA reviewing that call & downgrading the flagrant 2 to a flagrant 1," he tweeted. "Clearly I shouldn't have been thrown out of the game last night, BUT since I WAS thrown out, I ask for an explanation on the call & I get fined 25k???"
Barnes' foul was downgraded to a flagrant 1 after it was reviewed by the league.
Information from The Associated Press contributed to this report.