It's official -- Jake Paul has found Tommy Fury's replacement for his August 6 fight ... he's going up against former boxing champion Hasim Rahman's son.

25-year-old Paul shared the big news just minutes ago ... saying he will be stepping in the ring against 31-year-old Hasim Rahman Jr. at Madison Square Garden.

Paul praised his new opponent in the announcement ... saying HRJ is bigger, stronger, more experienced and has more knockout power.

"But it doesn't matter," Jake said Thursday. "They don't have my heart, don't have my drive & definitely don't have my team."

"August 6 I'm showing the world that Jake Paul is a bad motherf***er."

Paul is moving on from a boxing champ's brother to a boxing champ's son -- HRJ's dad is the former two-time world heavyweight champ, who posted a 50-9-2 record throughout his career.

As for HRJ, he is 12-1 ... most recently losing to James McKenzie Morrison via TKO.

Rahman Jr. and Paul have experience in the ring together, with Hasim praising the YouTube superstar after sparring in the past.

Folks wanting to see Jake fight Tommy may not want to hold their breath ... 'cause the two had a back-and-forth on Thursday, where Jake offered Fury $500k to fight after Aug. 6.

Naturally, Tommy declined.

Paul vs. Rahman Jr. will be part of the main event at MSG ... co-headlining with the Amanda Serrano and Brenda Carabajal fight.