Sean "The Cannon" Gannon is a Boston, Massachusetts police officer, and mixed martial arts (MMA) fighter. He gained notoriety on the Internet due to a widely distributed video of Kevin Ferguson "Kimbo Slice" losing to Gannon in an unsanctioned bare-knuckle street fight.

"Sean Gannon is the only man to defeat Kevin Ferguson 'Kimbo Slice' in his widely spread internet 'streetfight' videos. The fight took place inside a Boston gym and was the first time Slice took a streetfight outside of Florida. Gannon was proclaimed the victor after Slice couldn't answer a count of thirty after he was knocked down. The victory was controversial however with Gannon utilizing knees and chokes in the fight which were claimed to be illegal in the 'streetfight'. The fight also contained a lot of interference from both fighters' parties when Gannon began to choke and knee Slice. This notoriety lead him to be the focus of a front page article on the Boston Herald, and an internal Boston Police investigation into his off-hours activities.

The incident also caught the attention of the Ultimate Fighting Championship, who then promoted a fight between him and professional mixed martial arts fighter Branden Lee Hinkle, where Gannon ended up losing to Hinkle by TKO in the first round. Currently, Gannon has a professional MMA record of 1-1. He has not fought since losing to Hinkle in 2005. It was rumored that Gannon was arrested in Dubai for his part in a bar room brawl in late 2005.

Since signing a professional MMA contract with Elite XC Kimbo Slice has repeatedly expressed his interest in a rematch with Gannon even saying he would give his 'left testicle' for a rematch. In 2008 before Kimbo Slice's announced fight against Ken Shamrock and subsequent loss to Seth Petruzelli at Elite XC:Heat, there were rumours that Slice's next opponent would be a rematch with Gannon, this time inside a cage. However, the fight did not materialize. It is said that this was because Gannon's asking price was too high, as a result of his job with the Boston Police Force being at risk.