Will Smith slapped Chris Rock in the face on stage at the Oscars after the comic made a joke about the actor's wife Jada Pinkett Smith.
"Jada, can't wait for GI Jane 2," he said, in an apparent reference to her shaved hairdo - which is a result of the hair loss condition alopecia.
Smith walked on stage and struck Rock before returning to his seat and shouting: "Keep my wife's name out of your [expletive] mouth."
He later apologised on stage.
"I want to apologise to the Academy. I want to apologise to my all my fellow nominees," he said during a tearful acceptance speech for best actor.
The star picked up the first Oscar of his career for playing the father of tennis legends Venus and Serena Williams in King Richard.
"Art imitates life. I look like the crazy father, just like they said about Richard Williams. But love will make you do crazy things."
Rock's joke referred to 1997 film GI Jane, in which Demi Moore played the title role with a severe buzzcut.
The comedian looked stunned in the immediate aftermath of the incident, but told the audience: "That was the greatest night in the history of television."
The Los Angeles Police Department later told Variety that Rock had "declined to file a police report" following the event.
The Academy of Motion Picture Arts and Sciences, which organises the Oscars, tweeted that it "does not condone violence of any form".
Here in the Dolby Theatre in Los Angeles, there was a shocked reaction from journalists to Smith's run-in with Rock.
Reporters had been taking part in the traditional backstage press conference, where the winners come to answer questions after delivering their acceptance speeches.
But attention suddenly turned to our overhead screens and what was going on at the ceremony, as it became clear something serious was kicking off.
At first, it had seemed like a joke or some kind of set-up. Smith even appeared to laugh at first after Rock delivered his line about his wife looking like GI Jane.
Jada looked annoyed, but at this point it was still assumed this was all part of some kind of pre-planned routine.
Doubt began to creep in when Smith rose from his seat and hit Rock on the stage. Of course, both these guys are veterans of film and television, and would know how to carry out a fake stage slap. Except, everyone was starting to think, it didn't look that fake.
By the time Smith was sitting back in his seat and shouting to Rock to "keep my wife's name out your [expletive] mouth", it was clear this was no sketch. A professional like Smith would know better than to drop the F-bomb on stage during a live TV broadcast.
Viewers at home didn't hear the swearing. Broadcast network ABC had cut the live feed to avoid offending viewers at home.
Throughout the altercation, there was complete silence in the winners' room. Academy staffers looked just as shocked as the journalists did. "I thought they were doing a bit," one said to his colleague.
Rock, meanwhile, looked visibly shaken. But, realising he'd just made Oscars history, he made light of the situation, commenting: "That was the greatest night in the history of television."
Much like previous Oscars that were defined by their viral moments - such as Ellen's group selfie or the best picture winner mix-up in 2017 - this will now forever be the Oscars where Will Smith punched Chris Rock.
Pinkett Smith first talked about her hair loss struggle in an episode of her Facebook chat show, Red Table Talk, back in 2018.
She said: "I've been having issues with hair loss. And it was terrifying when it first started."
The Girls Trip star said she first suspected she had alopecia after "handfuls of hair" came loose in the shower.
"I was just like, 'Oh my God, am I going bald?' It was one of those times in my life where I was literally shaking with fear," she explained. "That's why I cut my hair and continue to cut it."
After the incident, Rock handed over the best documentary prize, which was the reason he was on stage.
According to The Hollywood Reporter's Scott Feinberg, a tearful Smith needed to be "pulled aside and comforted" by Denzel Washington and Tyler Perry during an advert break.
Presenting the next section, Sean "Diddy" Combs said: "Will and Chris, we're going to solve that like family. Right now we're moving on with love."
It was not the first time Rock has made a joke about Pinkett Smith at the Academy Awards. He hosted the 2016 event, when she was among the stars who boycotted the ceremony due to a lack of diversity in the nominations.
That year, he said: "Jada Pinkett Smith boycotting the Oscars is like me boycotting Rihanna's panties - I wasn't invited!"
Other winners at Sunday's ceremony included Jessica Chastain, who picked up best actress for The Eyes of Tammy Faye; Jane Campion, who won best director for The Power of the Dog; and Apple TV film Coda, which was named best picture.
Best supporting actress went to Ariana DeBose for West Side Story, while Troy Kotsur won best supporting actor for Coda, which also picked up best adapted screenplay.
Sci-fi epic Dune won six awards in total including technical categories including best visual effects and best cinematography.