The rapper, gamer and boxer launched his own drinks brand last year, which includes a hydration range in a bottle, which is essentially flavoured vitamin water, and an energy range in cans which contain high levels of caffeine.
Prime is extremely popular with children and some schools have banned the drink.
"And with Prime energy - that shouldn't be allowed in schools.
"With Prime energy I put it on the supermarkets. They're the ones selling it. Kids should not be allowed it."
A 330ml can of Prime energy contains 140mg of caffeine. A 250ml can of Red Bull energy drink contains 80mg of caffeine. A 500ml can of Monster energy contains 180mg of caffeine.
KSI – real name is Olajide 'JJ' Olatunji – created the brand with fellow YouTuber Logan Paul and it launched in Asda supermarkets in October 2022 before it became more widely available in December 2022, with an RRP of around £2 a bottle.
But due to a high demand it began selling at inflated prices of up to £100 a can.
KSI denied this was a marketing ploy, joking: "I wish!"
He added: "No, we tried to make enough and the demand was just too high.
"I remember being in the UK when we released it in the UK and it was like 'Oh it's already sold out.' "
An account on social media platform X [formerly Twitter] and app Prime Tracker UK informs fans which stores have been reported to have the drink in stock each day, and stock has risen in the past months.
KSI previously told Capital Radio's Roman Kemp: "I don't like the reselling. I hate the reselling. And so like, even when Asda tried to up their price to £2.50…
"I was tweeting them like 'nah, nah, nah, nah, this is out of order, this isn't Prime doing this, this is Asda' and instantly they brought it back down to £2. It was crazy man."
KSI rose to fame through his YouTube videos about gaming and then started to release music. His singles include Really Love and Don’t Play, and he unveiled his debut studio album Dissimulation last year.
He became a boxer after challenging British YouTuber Joe Weller to a fight in a social media 'feud'.
In 2021 he apologised for his past use of a transphobic slur. He quit Twitter at the time but has now returned to the social media platform.
He also apologised in 2017 for sexual harassment after an old video resurfaced of him making sexist and misogynistic comments to women at the 2012 Eurogamer Expo.
Watch: Doctors have raised concern about the high levels of caffeine in Prime energy