Floyd Mayweather Jr. will come out of retirement to meet UFC star Conor McGregor in an Aug. 26 boxing match that will feature two of the top-selling fighters in the world.

The two fighters both announced the fight Wednesday, after months of speculation about whether Mayweather would return at the age of 40 to face a mixed martial arts fighter who has never had a pro boxing fight.

Oddsmakers immediately made Mayweather a big 11-1 favorite in a fight that will take place in a boxing ring and be governed by boxing rules. It will take place at 154 pounds.

“It’s official,” Mayweather said on Instagram next to a video poster of both fighters.

“THE FIGHT IS ON,” McGregor tweeted several minutes earlier, posting a picture of himself next to one of Mayweather’s father, Floyd Sr.

Mayweather, who retired in September 2015 after winning all 49 of his pro fights, will face the Irish UFC superstar at the T-Mobile arena on the Las Vegas Strip. He had tweeted a picture of himself sparring in recent days to show he was already getting ready for the bout.

“This is really an unprecedented event,” said Stephen Espinoza, who heads Showtime Sports, which will handle the pay-per-view. “Really we haven’t seen anything in modern history that resembles it, it’s impossible to predict how many sales this will do.”

Espinoza said the fight came together quickly after McGregor and the UFC reached agreement last month on their end of the deal and Mayweather’s team pushed for the fight in recent days.

“All parties were motivated and reasonable and thrilled we could get everything done,” he said. “The sky’s the limit on this.”

Financial terms were not released, though Mayweather got the greater share of revenue when he fought Manny Pacquiao and is expected to have a similar percentage against McGregor.

Depending on pay-per-view sales, both fighters could earn huge purses, though probably not the $200 million or so Mayweather earned for Pacquiao.

“Everybody’s happy,” said Mayweather’s adviser, Leonard Ellerbe.