He went on to thank each of the group’s other members Simon Minter, Josh Bradley, Vikram Barn, Tobi Brown, Ethan Payne and Harry Lewis, saying they “deserve somebody who can give 100%”.
In a statement posted on social media by Sidemen, the group said they were “sad to share the news” that KSI had “decided not to continue”.
“We had an amazing time on this journey with him and we wish him all the best with his future endeavours,” they said.
“This came as a surprise to us and we know it will be an adjustment for you, but we’ve got lots of exciting stuff planned that we can’t wait to share.”
The collective have more than 140 million YouTube subscribers between them and have hosted charity football matches that have raised millions of pounds for charity.
The group has also launched its own fried chicken chain, clothing, alcohol and cereal.
KSI started making videos in 2008 and rose to fame as an online streamer. He has more than 18m subscribers to his personal YouTube channel.
He is also a musician, former boxer and recently bought a minority stake in Dagenham & Redbridge Football Club.