Former two-division world champion David Haye will end his retirement next month to take on entrepreneur Joe Fournier at the Staples Center in Los Angeles.
Haye retired three years ago as a professional at the age of 37, tasting a final defeat to Tony Bellew in their 2018 rematch.
The British legend hung up the gloves with a record of 28-4 having unified the cruiserweight division as well as winning the WBA title at heavyweight.
His most recent involvement in the sport was managing Derek Chisora.
But the 40-year-old is set to step back between the ropes on September 11 on the undercard of Oscar de la Hoya's own comeback against former UFC light heavyweight champion Vitor Belfort.
Fournier, 38, has won nine times since making his debut in 2015 but is more widely known for amassing a fortune in the fitness and nightclub industries. He will take on Haye in an eight-round bout at heavyweight.
"This whole fight between us, came into existence when at dinner with a group in Mykonos we were asked who would win in a fight between us," Haye said.
"I laughed, but out of respect for Joe's ego suggested it would be close, maybe a draw - whilst winking to Joe. Joe's straight-faced response was very different, he was deadly serious stating he would win in a fight today.
"Fast forward two weeks, I remain happily retired from boxing, with no intentions to make a traditional comeback to challenge the monsters of the division but am fit and ready to prove my point against my overconfident billionaire buddy.
"Whilst an unexpected challenge, since retirement I have remained in the gym and I am always mindful of what I put in my body meaning with just four weeks' notice, I am currently 10lbs lighter than my first fight against Tony Bellew some four years ago.
"This is not a comeback; this is about teaching Joe Fournier there are levels to the boxing game. One must stay in their lane or risk getting flattened."
Fournier said: "I'm here to make a statement. Like the business world, timing is everything in the fight game. I have immense respect for David's past achievements, but his time has passed.
"I'm younger, fitter and faster. He may have been world heavyweight champion, but that moment has gone, I am still learning the sport, coming into my prime. His demise will meet my rise and come September 11th the boxing world is in for a huge shock."