KSI responded with excitement, shouting: “Let’s go!” as he was showered in gold confetti.
Tonioli said in a farewell statement: “After three incredible series, I’ve sadly had to walk away from a job which has opened my eyes to so much incredible talent and step down as a judge on BGT.
“And whilst I look forward to filming a new series of Dancing with the Stars in the US, I will miss the chaos and joy that BGT brings, as well as all the wonderful people I’ve had the privilege of working with.
“They truly are a brilliant team who make brilliant television, and I know KSI will be a fantastic judge too, as he’s already proven this year.”
Tonioli joined Britain’s Got Talent in 2023, replacing David Walliams, 54.
KSI, who first served as a BGT guest judge during the 18th series earlier this year, proved popular with audiences for his enthusiasm and high energy.
His golden buzzer pick, Harry Moulding, went on to win the competition, securing the £250,000 prize and a place at the Royal Variety Performance.
Celebrating his return to the show, KSI said: “I’m so grateful and happy to be a part of the Britain’s Got Talent team for another season.
“I had such a good time last year and I can’t wait to see some more top talent. I’m full of energy, ready to go and can’t wait to make this the most entertaining BGT season ever! Let’s do this.”