Olajide Olayinka Williams "JJ" Olatunji (born 19 June 1993), better known as KSI, is an English YouTuber and rapper. He is a co-founder and member of the British YouTube group known as the Sidemen.

KSI registered his main YouTube account in 2009 and built a following posting gaming-commentary videos of the FIFA video game series. His YouTube content has diversified to include vlog and comedy style videos. As of March 2021, he has over 34 million subscribers and over 8 billion video views across his three YouTube channels.

KSI's 2020 debut studio album, Dissimulation, debuted at number 2 on the UK Albums Chart. His second studio album, All Over The Place, debuted at number 1. He has achieved fourteen top 40 singles on the UK Singles Chart, seven of them ending up in the Top 10 and five of them ending up in the Top 5. KSI featured in the British comedy film Laid in America (2016), and was the subject of KSI: Can't Lose (2018), a documentary film following the build-up to his first amateur boxing fight against British YouTuber Joe Weller. He was involved in two further boxing events against American YouTuber Logan Paul, the second of which was a professional bout. On 27 August 2022, he was involved in two professional crossover boxing bouts against British rapper Swarmz and Mexican boxer Luis Alcaraz Pineda on the same night. He is the CEO of Misfits Boxing and the co-owner of Prime Hydration, XIX Vodka and a restaurant chain known as Sides.