Britain's former world cruiserweight champion David Haye will take on Russia's WBA heavyweight champion Nikolai Valuev on 7 November.
The announcement ends speculation about who Haye's next opponent would be, following reports the Londoner had agreed a bout with Vitali Klitschko.
Haye, 28, is eight stones lighter and nine inches smaller than Valuev.
He said: "David only needed a slingshot and stone to flatten Goliath, my right hand generates more power than that."
Valuev, who is 7ft tall, has yet to be floored or knocked out in his 16-year professional career and has lost just once in 50 fights.
But Haye said: "I am thrilled to announce that my dream of becoming world heavyweight champion will be realised on 7 November, when I challenge the tallest and heaviest champion of all time.
"I can't wait to step into the ring and prove that size doesn't matter. Mark my words: I will be the first man to knock Nikolai Valuev out."
A venue for the contest has yet to be announced.
Cant see him knocking Valuev out myself
The announcement ends speculation about who Haye's next opponent would be, following reports the Londoner had agreed a bout with Vitali Klitschko.
Haye, 28, is eight stones lighter and nine inches smaller than Valuev.
He said: "David only needed a slingshot and stone to flatten Goliath, my right hand generates more power than that."
Valuev, who is 7ft tall, has yet to be floored or knocked out in his 16-year professional career and has lost just once in 50 fights.
But Haye said: "I am thrilled to announce that my dream of becoming world heavyweight champion will be realised on 7 November, when I challenge the tallest and heaviest champion of all time.
"I can't wait to step into the ring and prove that size doesn't matter. Mark my words: I will be the first man to knock Nikolai Valuev out."
A venue for the contest has yet to be announced.
Cant see him knocking Valuev out myself