Gibson's plea was part of a deal that sentences him to 36 months of probation and 52 weeks of domestic violence counseling.
Ex-girlfriend Oksana Grigorieva accused Gibson of punching her in the head and face during an argument in January 2010. Gibson later said he did so to protect their baby daughter, Lucia, whom Grigorieva was holding.
Oksana Grigorieva: I thought Mel would kill me
The accusation was followed by leaked recordings of Gibson yelling racist and sexist remarks at Grigorieva during their custody battle.
Gibson also alleged that Grigorieva tried to extort him, but the Los Angeles district's attorney office said they found insufficient evidence to charge her.