Sean Daniel O'Malley (born October 24, 1994) is an American professional mixed martial artist. He currently competes in the Bantamweight division of the Ultimate Fighting Championship (UFC), where he is the former UFC Bantamweight Champion. As of October 28, 2025, he is #2 in the UFC bantamweight rankings.

O'Malley trains in Glendale, Arizona, at The MMA Lab led by head trainer John Crouch, and is coached by his long-time friend, Tim Welch.

O'Malley was given the nickname "Sugar" by one of his MMA coaches in Montana early on in his career.

After achieving amateur records of 9–2 (MMA), 4–0 (kickboxing) and 2–0 (boxing), O'Malley stuck with MMA and debuted professionally, fighting his first five fights of his career in his native Montana, before leaving to fight in North Dakota and eventually fought for Legacy Fighting Alliance where he got a highlight-reel knockout over David Nuzzo.

After his win over Nuzzo, O'Malley earned a spot on Dana White's Contender Series 2 where he took on Alfred Khashakyan. O'Malley beat Khashakyan with a stunning knockout in the first round and was awarded a UFC contract.

O'Malley's promotional debut came against Terrion Ware on December 1, 2017, at The Ultimate Fighter 26 Finale. O'Malley won the fight via unanimous decision.

O'Malley next faced Andre Soukhamthath on March 3, 2018, at UFC 222. He won the fight by unanimous decision, despite injuring his foot in the third round. This fight earned both participants a Fight of the Night bonus.

O'Malley was expected to square off against José Alberto Quiñónez on October 6, 2018, at UFC 229. However, O'Malley announced that he was out of the fight on September 30 due to a potential Anti-Doping Policy violation. On October 25, O'Malley received hip surgery while waiting to receive judgment pending his potential violation. Subsequently, O'Malley was suspended for six months by the NSAC after testing positive for ostarine. He became eligible to return as of March 2019.