Matt Barnes allegedly attacked Derek Fisher after he discovered the New York Knicks' coach is in a romantic relationship with his estranged wife.

Fisher has been dating Gloria Govan for "a few months." Fisher filed for divorce from his wife Candace in March.

When Barnes learned that Fisher was at his former home with Gloria, he drove 95 miles to LA to “beat the s–t” out of Fisher, according to sources.

“Derek was in Gloria’s backyard with about 10 people having a bonfire on Saturday,” said a source. “Derek’s separated from his wife and there’s a relationship with [Gloria].”

Barnes and Fisher were once teammates with the Los Angeles Lakers.

“Barnes was in Santa Barbara [at Memphis Grizzlies training camp] and heard that Fisher was in his house. He went crazy. He got in his car and went to the house and went after Fisher.”

A source close to Fisher confirmed the fight but said, “Matt came after Derek but he only had a few scratches … Derek’s not going to press charges, he’s going to let it go.”

The Knicks tweeted on Monday that Fisher was “not at practice due to personal reasons — expected back tomorrow.”