Conor McGregor says he has retired from fighting - for the third time in four years.
Ireland's former two-weight UFC champion, 31, has a record of 22 wins and four defeats.
His last fight, in January 2020, saw him beat American fan favourite Donald 'Cowboy' Cerrone in just 40 seconds in Las Vegas.
He has previously announced his retirement in 2016 and 2019 before twice returning to the octagon.
McGregor also tried boxing, losing to former five-weight world champion Floyd Mayweather in August 2017.
"Hey guys I've decided to retire from fighting," he said on Twitter on Sunday.
"Thank you all for the amazing memories! What a ride it's been!"
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He later told ESPN he was "bored of the game".
McGregor added: "The game just does not excite me, and that's that. All this waiting around. There's nothing happening.
"I'm going through opponent options, and there's nothing really there at the minute. There's nothing that's exciting me."
Speaking after UFC 250 in Las Vegas on Saturday, the organisation's president Dana White said he had not been given advance warning of McGregor's announcement.
"One of my people grabbed me and showed it to me on social media - that's Conor, that's how Conor works and does things," said White.
"Conor McGregor's not going to need some money any time soon. He loves the fight, he's passionate about it and he has an incredible fighter IQ. But whatever his reason is, when you say you want to retire, don't worry about it. Do your thing, man."
White added: "We're in a pandemic, the world is a crazy place right now. If these guys want to sit out and retire right now, or if anybody feels uncomfortable in any shape or form about what's going on, you don't have to fight - it's all good. So if that's what's Conor's feeling right now, Jon Jones, Jorge Masvidal, I feel you."
UFC events are taking place without fans because of the coronavirus pandemic and last month McGregor suggested he would like to compete in a behind-closed-doors event.
Highs and lows of McGregor
McGregor signed for the UFC in 2013 as a two-division Cage Warriors champion, and went on to capture the UFC titles at featherweight and lightweight, becoming the first man to simultaneously hold UFC titles in two weight classes.
Those titles were subsequently stripped by the UFC after he failed to defend his belts.
After his first 'retirement' in 2016, he returned in October 2018 to unsuccessfully challenge for the UFC lightweight title against Khabib Nurmagomedov at UFC 229.
Earlier this year, he displayed one of the most spectacular performances of his career to defeat Cerrone - his first win inside the octagon since taking his second world title in 2016.
In May, McGregor declared himself as the second greatest mixed martial arts fighter of all time behind Anderson Silva, adding he would "easily" surpass the Brazilian before retiring.
However, McGregor's time in mixed martial arts has also been marred by controversy.
In 2018, he was ordered to have anger management training and perform five days of community service by a court in return for criminal charges being dropped after he had attacked a bus containing rival UFC fighters.
Video footage appeared to show McGregor throwing a railing at a bus carrying Nurmagomedov and a number of other UFC fighters.
In March 2019, McGregor was arrested in Miami for allegedly smashing a fan's phone as they tried to take pictures of him. The charges were later dropped, although McGregor settled a civil lawsuit with the victim out of court.