Championship Rountree made his promotional debut on July 8, 2016 at UFC The Ultimate Fighter 23 Finale for the TUF 23 Light Heavyweight title against Andrew Sanchez. He lost the fight via unanimous decision. Rountree next faced Tyson Pedro at UFC Fight Night: Whittaker vs. Brunson on November 27, 2016. He was submitted via rear-naked choke in round one. On his third fight Rountree faced Daniel Jolly at UFC Fight Night: Bermudez vs. The Korean Zombie on February 4, 2017. He won via knockout in the first round. Rountree faced Paul Craig on July 16, 2017 at UFC Fight Night: Nelson vs. Ponzinibbio. He won the fight via knockout in the first round. Rountree was expected to face Gökhan Saki on December 30, 2017 at UFC 219. However, Saki was forced to pull out, citing knee injury, and was replaced by Michał Oleksiejczuk. Rountree lost the fight by unanimous decision. However, after the fight, it was revealed that Oleksiejczuk had tested positive for clomiphene, an anti-estrogenic substance. As a result, the Nevada Athletic Commission (NAC) officially overturned the result of the fight to a no contest. The bout between Rountree and Gökhan Saki eventually took place at UFC 226 on July 7, 2018. Rountree won the fight via knockout in round one. This fight earned him the Performance of the Night award. Rountree faced promotional newcomer Johnny Walker on November 17, 2018 at UFC Fight Night 140. He lost the fight via knockout in the first round. Rountree faced Eryk Anders on April 13, 2019 at UFC 236. He won the fight via unanimous decision. Rountree faced Ion Cuțelaba on September 28, 2019 at UFC Fight Night 160. He lost via TKO in the first round. Rountree was expected to face Sam Alvey on March 28, 2020 at UFC on ESPN: Ngannou vs. Rozenstruik. Due to the COVID-19 pandemic, the event was eventually postponed. Despite talks of retiring after the scrapped bout with Alvey, Rountree signed a contract extension with the UFC in October 2020. Rountree faced Marcin Prachnio on January 24, 2021 at UFC 257. He lost the fight via unanimous decision. Rountree faced Modestas Bukauskas on September 4, 2021 at UFC Fight Night 191. He won the fight via TKO in round two after catching Bukauskas with an oblique kick, marking the first time the technique had been used to finish a fight in the UFC. Rountree faced