The British YouTuber, who has more than 20 million subscribers, won a six-round cruiserweight bout against fellow YouTube personality Logan Paul in Los Angeles on Saturday.
After the match Bieber, who was in attendance, maintained Paul had been the “better fighter”.
Appearing on Good Morning Britain, KSI said that if Bieber, 25, wanted to fight, they would “make it happen”.
However, he warned “there’s no point” because he would “destroy” the Sorry chart-topper.
Host Piers Morgan asked whether KSI would fight Bieber.
KSI said: “If Justin Bieber actually wants to fight me then yeah, obviously we will make it happen.
“Justin Bieber, I would destroy him. There’s no point. There would be nothing.
“He doesn’t know how to fight. He doesn’t know how to do anything combat sports. Let him just stick to singing.”
Asked by co-host Susanna Reid whether he thought it would be an unfair fight, KSI replied: “Obviously. Haven’t you seen? I destroyed Logan Paul. Why would I fight Justin Bieber?”
KSI, 26, from Watford, Hertfordshire, beat 24-year-old US star Paul at the Staples Centre in a split decision – 55-56 57-54 56-55.
Paul, who has 19.9 million YouTube subscribers, did not touch gloves with his rival at the end of the match.
The men first fought last year in Manchester, but a “majority draw” was declared after six rounds.
Their bout at Manchester Arena sold out to 20,000 fans and drew an audience of 2.5 million live viewers and more than 19 million subsequent views, making it the biggest internet event in history.
Bieber has also recently engaged in fighting talk, earlier this year challenging Tom Cruise to a UFC-style cage fight on Twitter.
The singer tweeted the Mission: Impossible star, daring him to face him in the mixed martial arts sport’s Octagon arena.
Good Morning Britain is on ITV weekdays from 6am. With additional reporting by Press Assocation.