Tommy Fury has promised to knock out KSI in front of a home crowd after the pair announced they would fight.
The pair have been verbally sparring for some time with both camps blaming the other for the negotiations dragging.
A deal has been agreed and they will fight at the AO Arena in Manchester on 14 October.
The weight and number of rounds for the bout was not announced on Sunday night but Fury, posting about the fight on social media, was in confident mood.
It will be the first time Fury has boxed since he beat Jake Paul in February.
KSI, who shot to fame as a YouTuber before donning the gloves for the first time in 2019 to face Logan Paul, who he has since become business partners with.
Fury, the half-brother of unbeaten heavyweight world champion Tyson, has won all nine of his fights.
KSI also announced that his Prime energy drink business partner Logan Paul will also be fighting at the same event.
The two social media stars fought each other in 2019, with KSI winning in a split decision.
The fight will not be sanctioned by the British Boxing Board of Control as KSI does not hold a professional licence in the UK – instead the bout will be held under the auspices of the Professional Boxing Association and will not count towards any professional records.