LAS VEGAS — Floyd Mayweather Jr. said Wednesday he will come out of retirement to face a UFC star, Conor McGregor, in a boxing match Aug. 26.

Mayweather, who retired in September 2015 after winning all 49 of his pro fights, will face a mixed martial arts fighter who has never been in a scheduled 12-round fight at the MGM Grand arena. The fight will take place in a boxing ring and be governed by boxing rules.

“It’s official,” Mayweather said on his Instagram account 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 will fight at the relatively advanced age of 41 in a bout that McGregor has been pushing for months. It finally came together over the last few days and Nevada boxing officials on Wednesday approved the date for a Mayweather Promotions bout.

Though Las Vegas oddsmakers have made Mayweather a 10-1 favorite, the thought of the fight has excited many in the MMA world. It also has intrigued some in boxing, though most dismiss McGregor’s chances under boxing rules against one of the greatest defensive fighters in history.

Even if the actual bout may not shape up as a great matchup, the run-up to the fight will. Both fighters are noted for their ability to sell fights, and have exchanged in trash talking and more to promote their bouts.

The fight will be televised on pay-per-view on Showtime. It comes less than a month before Gennady Golovkin and Canelo Alvarez meet in a highly anticipated fight on pay-per-view.