Olajide Olayinka Williams "JJ" Olatunji (born 19 June 1993), known professionally as KSI, is an English social media influencer, professional boxer, and musician. He is a co-founding member of YouTube group the Sidemen, the CEO of Misfits Boxing, and the co-owner of Prime Hydration, XIX Vodka, Sidemen Clothing, restaurant chain Sides, cereal brand Best Breakfasts, and snack brand Lunchly.

He registered his main YouTube channel in 2009 and built a following through his gaming commentary videos of the FIFA video game series. His content later diversified to include vlogs and comedy-style videos. As of October 2022, he had over 41 million subscribers and over 10 billion video views across his three YouTube channels. In 2021, he was listed among Forbes 30 Under 30 for the Europe Entertainment category. He has also ventured into acting, featuring in the British comedy film Laid in America in 2016. He was the subject of KSI: Can't Lose in 2018, a documentary film following the build-up to his first boxing match, and KSI: In Real Life in 2023, an Amazon Prime Video documentary film following his life from June 2021 to October 2022.

Creating rap music since 2011, he released his debut studio album, Dissimulation, in 2020, which debuted at number 2 on the UK Albums Chart. His second studio album, All Over the Place, was released in 2021 and 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.

He was involved in a white-collar boxing match against Joe Weller in February 2018. Two further boxing events against American YouTuber Logan Paul took place, the second of which was a professional bout in which he won via split decision in November 2019. In August 2022, he fought two people in the same night, defeating rapper Swarmz and professional boxer Luis Alcaraz Pineda. In 2023, he fought FaZe Temperrr in January, Joe Fournier in May, and Tommy Fury in October, which he lost via unanimous decision.

Olajide Olayinka Williams Olatunji was born on 19 June 1993 in London and was raised in Watford, Hertfordshire. His father, Olajide Olatunji, was born in Ibadan, Nigeria. His mother, Olayinka Olatunji (née Atinuke), is from Islington, London. He and his parents are from the Yoruba ethnic group. He was educated at Reddiford School in Pinner and Berkhamsted School in Berkhamsted, where he met future collaborator and Sidemen member Simon Minter.

KSI's younger brother, Deji Olatunji, is also a YouTuber previously known as ComedyShortsGamer, and professional boxer. The brothers ranked first and second respectively as the "UK's Most Influential YouTube Creators" by Tubular Labs in 2015.

KSI registered his first account on YouTube under the username "JideJunior" in 2008 while in his early teens. He registered his current YouTube account on 24 July 2009 under the name KSI Olajide BT, where he uploaded gaming–commentary videos of the FIFA video game series from his bedroom at his parents' house in Watford. "KSI" stands for Knowledge Strength Integrity, which was taken from a Halo clan.

He dropped out of sixth form college to pursue his YouTube career once he was earning steady monthly revenue from his uploads. In an interview in 2014, he recounted asking his teacher whether he should drop-out. His teacher asked how much KSI was making from YouTube, to which KSI answered, "around £1,500 a month", which was more than his teacher earned. His parents initially disapproved, but later became supportive and featured in a number of his videos. KSI later uploaded more vlog-style content and played a variety of games and in 2012, the channel reached a million subscribers.

KSI's rise to fame was not without controversy, with him being widely criticised for his self-described 'rape-face', a recurring joke on his channel during 2012 and 2013. He was the centre of a controversy following third-party allegations of sexual harassment of female staff at a Eurogamer event in 2012, that went so far as accusing him of sexually assaulting a spokesmodel Brandy Brewer, despite her tweeting that she consented to the motorboat, writing, "it's called comedy……relax". As a result, Microsoft cut ties with KSI and he was banned from Eurogamer events. KSI subsequently apologised "for any offence the video of 15 months ago may have caused in the short time it was on his YouTube channel, references to it since and subsequently used by other people" and stated his desire to move on from the incident and "be judged on the great content and value he gives to brands and partners, without controversy."

In October 2013, KSI signed with Maker Studios' sub-network, Polaris. Since 19 October 2013, KSI has been part of the British YouTube group known as the Sidemen. The group produces online videos, most often consisting of challenges, sketches and video-game commentary, as well as selling exclusive Sidemen merchandise.

In July 2014, KSI and Comedy Central UK launched KSI: Demolished, a five-episode comedy roast web series in which British comedians such as Kojo, Michelle de Swarte, Bobby Mair, and Alex Lowe, take turns to roast KSI. In August 2014, KSI was the 4th most popular personality among U.S. teenagers, according to a survey done by Variety. On 18 August 2014, KSI teamed up with Endemol for a new sports show called Rule'm Sports. In October 2014, KSI signed with MLG.tv, a streaming media service focused on gaming and esports, as part of the roster of live streamers. In September 2015, KSI was ranked 15th on Business Insider's list of "The 15 most popular YouTubers in the world".

On 4 August 2017, KSI tweeted that he would be leaving the Sidemen, citing conflicts with fellow member, Ethan Payne. Shortly thereafter, he released a number of diss track videos criticising members of his then-former group, to which most of the members responded back with their own. Later that month, KSI released a video in which he claimed that he was being deported from the United States for receiving an incorrect visa. In November 2017, KSI released a video discussing whether his supposed falling out with the Sidemen was real or fake, saying, "[the drama] wasn't entirely real but wasn't entirely fake either."

In 2019, he was ranked second by The Sunday Times in its list of the top 100 UK influencers. In 2020, The Times named KSI to be Britain's biggest influencer. In April 2021, he was listed among Forbes 30 Under 30 Europe 2021 Entertainment list. In September 2023, Forbes ranked KSI as the second top creator of 2023, earning an estimated $24 million. In July 2024, KSI appeared in a YouTube video challenge by American YouTuber MrBeast titled "50 YouTubers Fight for $1,000,000".

Having written and produced comedy–rap songs for his YouTube channel in 2011, KSI released his debut single, "Lamborghini", featuring British grime MC P Money, on 23 March 2015, with Dcypha Productions. The song charted at number 30 on the UK Singles Chart, and It dropped to number 68 and number 96 over the following two weeks, before dropping out of the top 100.

"Keep Up", the title song for KSI's debut extended play of the same name, featuring Jme, was released on 13 November 2015, reaching number 45 on the UK Singles Chart. The full EP was released on 8 January 2016, through Island Records, debuting at number 13 on the UK Albums Chart and number one on the UK R&B Albums Chart.

On 29 April 2016, KSI released "Goes Off", featuring Mista Silva, as the first single from his second EP titled Jump Around. The EP's second single, "Friends with Benefits", featuring MNDM, was released on 29 July 2016, reaching number 69 on the UK Singles Chart. The full EP was released on 28 October 2016 through Island Records. One of the EP's songs, "Touch Down", featuring rapper and singer Stefflon Don, appeared on the soundtrack of the 2017 film Baywatch.

On 23 June 2017, KSI independently released "Creature", which reached number 100 on the UK Singles Chart. "Creature" was the first single from KSI's third EP, Space, which was released independently on 30 June 2017. On 6 October that year, KSI released his fourth EP, Disstracktions, which includes diss tracks against fellow Sidemen members W2S and Behzinga. The EP reached number 31 on the UK Albums Chart and number one on the UK R&B Albums Chart. One week before its release, KSI announced that the EP would be his "final release" with Island Records and that he would be releasing music independently.

On 2 February 2018, KSI released "Uncontrollable", featuring Big Zuu. The song played during KSI's ring walk for his boxing fight against Joe Weller and charted at number 89 on the UK Singles Chart. KSI released "On Point" on 17 August 2018, which was played during his ring walk for his second boxing bout.

On 12 April 2019, KSI released a collaborative album with Randolph titled New Age. It debuted at number 17 on the UK Albums Chart and at number one on the UK R&B Albums Chart.

On 4 November 2019, it was announced that KSI had signed with RBC Records and BMG to "take [his] music to the next level" and "further develop his music career in the US and internationally". In addition to managing KSI's future releases, it was confirmed that the label would be administering and reissuing KSI's independent catalogue recordings. That day, it was confirmed that KSI had already started recording his debut studio album. On 8 November 2019, "Down Like That", featuring Rick Ross, Lil Baby and S-X, was released as the album's lead single. It was performed live by the three featured artists as KSI's entrance music for his boxing rematch against Logan Paul.

The song peaked at number 10 on the UK Singles Chart and it has been certified silver by the British Phonographic Industry (BPI) for sales of 200,000 units in the country. On 15 December KSI, Miniminter, TBJZL, and Zerkaa released a Christmas themed single titled "The Gift" under the Sidemen banner featuring S-X which charted at number 77 in the UK. A further three singles preceded the album: "Wake Up Call", featuring Trippie Redd, which debuted at number 11 on the UK Singles Chart; "Poppin", featuring Lil Pump and Smokepurpp, which charted at number 43 in the UK; and "Houdini", featuring Swarmz and Tion Wayne, which debuted at number six on the UK Singles Chart and received a silver certification from the BPI.

KSI's debut studio album, titled Dissimulation, was released on 22 May 2020. The album debuted at number two on the UK Albums Chart and further charted in 15 other countries. It was the UK's best-selling debut album by a British artist in 2020 and it has been certified gold by the BPI for sales of 100,000 units in the country. The album spawned two further UK top 40 singles, "Cap", featuring Offset, and "Killa Killa", featuring Aiyana-Lee. KSI was scheduled to perform at Parklife, Longitude and Reading and Leeds music festivals in 2020 to promote Dissimulation, but these were later cancelled due to the COVID-19 pandemic. KSI featured on "Lighter" by British DJ and record producer Nathan Dawe. The song peaked at number three on the UK Singles Chart and became one of the UK's best-selling songs of 2020. It has been certified platinum by the BPI for sales of 600,000 units in the UK and it was nominated for Best British Single at the 2021 BRIT Awards. KSI also featured on "Loose" by Zimbabwean-British artist S1mba, which peaked at number 14 on the UK Singles Chart.

On 23 October 2020, KSI released the first single from his second album, "Really Love" featuring Craig David and Digital Farm Animals, which peaked at number three on the UK Singles Chart and has been certified gold by the BPI. On 15 January 2021, KSI released the album's second single, "Don't Play" with Anne-Marie and Digital Farm Animals. The song debuted at number two on the UK Singles Chart, becoming KSI's highest-charting single in the UK to date, and has been certified platinum by the BPI. On 12 March 2021, KSI released the album's third single, "Patience" featuring Yungblud and Polo G, which debuted at number 3 on the UK Singles Chart and has been certified silver by the BPI. On 26 April 2021, KSI announced via his social media the release of his second album All Over the Place, which was released on 16 July that year. It debuted at number 1 on the UK Album Charts, giving KSI his first-ever chart-topper, and has been certified gold by the BPI for surpassing 100,000 sales in the country. On 18 June 2021, KSI released the album's fourth single, "Holiday", which was only featured on the platinum edition of the album. The song debuted at number 2 on the UK Singles Chart, making it KSI's highest-charting solo single to date. On 6 August 2021, KSI released the album's fifth single, "Lose" with American rapper Lil Wayne, and was the lead single for the deluxe edition of the album, which was released on 27 August. The single peaked at number 18 on the UK Singles Chart and also debuted at number 86 on the Billboard Hot 100, becoming KSI's first ever single to enter that chart.

On 6 May 2022, KSI featured on "Locked Out" by S-X. The single is S-X's third single for his debut album. "Locked Out" became S-X's first single to chart in the UK Official Charts which peaked at number 53. On 30 May, it was announced that KSI had signed with Atlantic Records and Warner Music UK "following his huge collaboration with Anne-Marie on Don't Play". On 5 August, KSI released "Not Over Yet" featuring Tom Grennan. "Not Over Yet" charted at number four on the UK Single Charts, and debuted at number one on The Official Big Top 40. The single marked the first record under Atlantic Records. A remix featuring Headie One and Nines released on 28 August and served as KSI's ring walk tune in his bout against Luis Alcaraz Pineda on 27 August. On 30 September, KSI released "Summer Is Over." On 5 December, KSI, TBJZL, and Vikkstar123 released a Christmas-themed single titled "Christmas Drillings" under the Sidemen banner featuring Jme, which charted at number 3 in the UK. On 27 January 2023, KSI released "Voices" featuring Oliver Tree. On 13 May, KSI released "Easy" with Bugzy Malone and R3HAB which served as KSI's ring walk tune in his bout against Joe Fournier on the same night.

On 31 August, KSI performed a two-part concert at MF & DAZN: X Series 17 in which he previewed two unreleased singles "Low" and "Thick of It" featuring Trippie Redd. On 3 October, KSI released both songs as a double-single release, the latter of which is subjected to negative reception and mockery.

Supposed hostilities between KSI and British YouTuber Joe Weller began towards the end of 2017. Following public disagreements on Twitter and diss-tracks between the pair, they announced that they would host an amateur boxing match on 3 February 2018 at Copper Box Arena, London, to settle the feud. During the announcement, the pair confronted each other, with KSI mocking Weller's struggles with depression and his use of antidepressants for which he later apologised.

The fight was won by KSI 1 minute and 37 seconds into the third round by way of technical knockout. KSI expressed his respect for Weller after the fight for being "way harder, way tougher than I thought," and praised him for his commitment to raising awareness for mental health, before calling out American YouTuber Logan Paul, his brother Jake Paul, and retired footballer Rio Ferdinand.

On 24 February 2018, it was announced that KSI would be fighting Logan Paul in a white-collar boxing amateur boxing match. The fight ended as a majority draw, with two judges scoring the fight even at 57–57 and a third judge scoring 58–57 in favour of KSI.

A rematch between KSI and Paul was held on 9 November 2019 at the Staples Center, Los Angeles, and was promoted by Eddie Hearn. Unlike the first fight, the rematch was a professional fight without the use of headgear. KSI was trained by professional boxers Viddal Riley and Jeff Mayweather, uncle of Floyd Mayweather Jr. KSI defeated Paul via split decision, with two judges scoring the bout 56–55 and 57–54 in favour of KSI, and a third judge scoring the fight 56–55 in favour of Paul.

On 1 July 2022, it was announced that KSI vs Alex Wassabi was scheduled to take place on as the headliner to MF & DAZN: X Series 001 27 August 2022; however, due to a major concussion, it was announced on 6 August that Swarmz would be Wassabi's replacement. This marked the first event under KSI's promotion company Misfits Boxing.

On 16 August, it was announced that KSI will be facing a second opponent, thus billing the event "2 Fights 1 Night." His initial second fight was scheduled to be against the Bulgarian boxer Ivan Nikolov, who was pulled from the fight due to Nikolov having multiple white supremacist and Neo-Nazi tattoos. On 20 August, the Mexican professional boxer Luis Alcaraz Pineda was announced as KSI's second opponent on 27 August. KSI defeated Swarmz via 2nd round KO and won the vacant MFB cruiserweight title. KSI then defeated Alcaraz Pineda via 3rd round KO and retained his MFB cruiserweight title.

On 19 November, it was announced that KSI would face Dillon Danis as the headliner of MF & DAZN: X Series 004 on 14 January 2023, at Wembley Arena, London, England. However, on 4 January, Danis withdrew due to lack of preparation, a lack of a coach, and issues with the contracted weight. It was announced that KSI would defend his MF cruiserweight title against Brazilian YouTuber and FaZe Clan founder FaZe Temperrr, replacing Danis within a weeks notice. KSI defeated Temperrr via 1st round KO and retained his MFB cruiserweight title.

On 22 March, it was announced that KSI would face businessman and professional boxer Joe Fournier as the headliner of MF & DAZN: X Series 007 on 13 May at Wembley Arena in London, England. KSI originally defeated Fournier via 2nd round KO and retained his MFB cruiserweight title; however, controversy arose after replays showed KSI landing an illegal elbow strike during the finishing sequence. On 15 May, Fournier appealed the decision, and the Professional Boxing Association called for both fighters to make their cases, and a decision will be made by 19 May at the latest. On 19 May, the Professional Boxing Association officially ruled the bout a no-contest.

On 28 July, it was announced that KSI and Logan Paul would both headline MF & DAZN: X Series 10 – The Prime Card on 14 October at Manchester Arena in Manchester, England. On 30 July, it was announced that KSI would in fact be facing Fury. Fury defeated KSI via a majority decision. On 15 October, the results were changed to a unanimous decision victory for Fury due to judge Rafael Ramos, who had previously scored the bout 57-57 on the scorecard, which was mathematically incorrect. The event generated 1,300,000 pay-per-view buys, making it one of the largest boxing events in 2023.

After the bout had concluded, KSI lashed out at Fury and insisted that the result was a "robbery." Many online and attendees sided with KSI, beveling that he did enough to win the fight, including English sports promoter Eddie Hearn, who was in attendance.

On 19 October, KSI submitted an appeal to the Professional Boxing Association (PBA). On 1 December, the PBA announced on Twitter that they had rejected the appeal, stating there were "no grounds" to overturn the result. Despite the result, KSI's manager Mams Taylor stated that the appeal is still ongoing and that they are moving into the second of three possible stages.

On 2 June 2024, KSI confirmed on Twitter that he would return to the ring in August after a 10-month break from the Fury loss in October of last year. Reports began to state that KSI was in negotiations to face former unified light welterweight champion Amir Khan. On 4 June, Khan confirmed the negotiations. However, on 8 June KSI confirmed in a YouTube video that Khan will not be his opponent but potentially for later in the year. In that video, KSI also confirmed he will be fighting on 31 August 2024, in Dublin, Ireland, and challenged American internet personality Slim Albaher and American mixed martial artist Anthony Taylor to a 2 vs. 1 outnumbered tag team match. On June 30, 2024 the bout between KSI, Albaher and Taylor was confirmed. However, fans began to speculate if the bout had been cancelled following KSI deleting the bout announcement across his social media. On 1 August, KSI confirmed his withdrawal in a Twitter post due to an injury sustained.

In 2016, KSI and Caspar Lee starred in the British direct-to-video comedy film Laid in America. The film was written and directed by Sam Milman and Peter Vass and was produced by The Fun Group LLC and Max Gottlieb. The film was released direct-to-video by Universal Pictures on 26 September 2016. Jason Best of Movie Talk gave the film 1 out of 5 and called it "Crass and witless."

In 2018, KSI released a documentary film titled KSI: Can't Lose. The film reveals the behind-the-scenes story of what led KSI to become the YouTube champion of the world after his boxing debut fight against fellow YouTube star Joe Weller and how KSI will use what he learned to fight Logan Paul. The film was produced by fellow YouTuber Callux's film studio, After Party Studios, and was distributed by Sony Pictures Home Entertainment.

KSI makes a brief appearance in the Amazon Original sports docuseries All or Nothing: Arsenal, which documented the club by spending time with the coaching staff and players behind the scenes both on and off the field throughout their 2021–22 season.

In 2023, KSI released a documentary called KSI: In Real Life, produced by Amazon Prime Video and Louis Theroux, focusing on KSI's life and career. Theroux stated that it would be "an opportunity to see another side of JJ, understanding how he got where he is, the world of a premium online content creator today, and a man who has made it his brand never to lose, as he takes on the music business."