The former heavyweight champion was asked to help out by 50 Cent after some major beef between the pair over the last few weeks.
"I'm gonna teach him how to bite somebody's ear. Yeah, that's right," he said in a video on Instagram.
"I'm gonna teach him every dirty trick in the book to knock you out. Because I'm not going to teach him how to run."
Soulja Boy is going to be trained by another former boxing champ, Floyd Mayweather, who announced the bout on Instagram.
The three-round fight will take place in Las Vegas but a date hasn't been announced.
The grudge started after Soulja Boy liked and left a comment below a picture on the profile of Karrueche Tran, Chris Brown's ex-girlfriend.
The pair posted several videos and pictures mocking and threatening each other as the argument heated up.
Soulja Boy posted about the fight but has since deleted it from Instagram.
He said: "It's going down! Signed my contract I'm leaving the fight with $1,000,000.
"I got the best ever my big bro @floydmayweather training me damn."
Chris Brown told his Instagram followers that it was time to stop talking.
"Now that we have your undivided attention, boxing match set," the singer wrote.
"Legally. Man to Man. No more dissing and no more talking. We in the ring with it. Challenge accepted."
50 Cent has backed Soulja Boy for the win and says he'll be placing a $100,000 (£80,619) bet on the outcome.
Chris Brown, 27, and Soulja Boy, 26, have been criticised by fans and other celebrities for their public feud.
Soulja Boy has even been accused of using the match as a tactic to save his flagging music career.
The exact venue and date of the fight is still to be confirmed but according to the flyer it will be a pay-per-view event.
Soulja Boy has already started training according to Snapchat posts.