In August 2014, Bhullar had his first amateur fight as a mixed martial artist. He turned pro in November of that year fighting with the Battlefield Fight League. Over the next few years, Bhullar competed exclusively in his native Canada and remained undefeated with a record of six wins. Arjan won the vacant BFL Heavyweight Title with a victory over Blake Nash on October 17, 2015. He has defended the title twice. Ultimate Fighting Championship In May 2017, Bhullar signed with the Ultimate Fighting Championship becoming the first fighter of Indo-Canadian descent to sign with the company. Bhullar made his promotional debut at UFC 215 on September 9, 2017, against Luis Henrique. He won the fight via unanimous decision. Bhullar's next fight was against Adam Wieczorek on April 14, 2018, at UFC on Fox 29. After controlling the first round with his wrestling, Bhullar would eventually get caught with an omoplata early in the second round, officially losing the fight via submission at 1:59 of round 2. For his third fight with the promotion, Bhullar faced Marcelo Golm on October 27, 2018, at UFC Fight Night 138. He won the fight via unanimous decision. Bhullar next faced Juan Adams on May 4, 2019, at UFC Fight Night 151. He won the fight by unanimous decision. The fight marked the last fight of his prevailing contract with the UFC. ONE Championship On July 3, 2019, it was announced that Bhullar signed a deal with Asia-based MMA promotion ONE Championship, and was expected to make his promotional debut against Mauro Cerilli at ONE Championship: Dawn of Heroes on August 2, 2019. However, Cerilli pulled out of the bout due to staph infection and the fight was cancelled. The fight eventually took place at ONE Championship: Century on October 13, 2019, with Bhullar winning by unanimous decision. ONE Championship Heavyweight Championship On February 1, 2020, it was announced that Bhullar would be challenging Brandon Vera for the ONE Heavyweight World Championship at ONE Championship: Code of Honor on May 29, 2020, but the duel was canceled due to the COVID-19 pandemic. The bout was rebooked to take place at ONE Championship: Dangal on May 15, 2021. He defeated Vera via second-round technical knockout to win the ONE Heavyweight World Championship, becoming the first fighter of Indian descent to win an MMA world title. After a contract dispute, Bhullar eventually signed a new contract