Arnold Billy Allen (born 22 January 1994) is a British professional mixed martial artist. He currently competes in the Featherweight division of the Ultimate Fighting Championship (UFC). As of 20 June 2026, he is #5 in the Meta UFC featherweight rankings.

Born and raised in Suffolk, Allen began training in mixed martial arts as a teenager and took part in several amateur fights before turning professional in 2012.

He made his professional debut competing as a featherweight for various regional promotions across Great Britain, including a stint in Cage Warriors. He was able to compile a record of 9–1 in his first three years along the way.^{[citation needed]}

After his TKO stoppage of Paul Cook in November 2014, Allen signed with the UFC.

Allen made his promotional debut on 20 June 2015 as a short notice replacement filling in for an injured Mike Wilkinson against Alan Omer at UFC Fight Night 69. After probably dropping the first two rounds, Allen won the fight via submission in the third round. He was also awarded a Performance of the Night bonus.

Allen next faced Yaotzin Meza on 27 February 2016 at UFC Fight Night 84. He won the fight by unanimous decision.

Allen was expected to face Mirsad Bektic on 8 October 2016 at UFC 204. However, due to injury, Allen withdrew from the fight and was replaced by Russell Doane.

Allen faced Makwan Amirkhani on 18 March 2017 at UFC Fight Night 107. He won the fight by split decision.