In September 2018, Saint Denis stood out among thirty fighters during a test organized by Daniel Woirin, a French coach renowned for having won three belts at
UFC and two at
Strikeforce with MMA champions
Anderson Silva,
Dan Henderson and
Lyoto Machida. Saint Denis, ending his contract with the
army, decided to embark on a career as a professional
MMA fighter early 2019. He gave himself two years to break through to the highest level in MMA. Two years was the time he had with his savings to train full time and get signed by the UFC. Then the pace of the fights slowed down due to the
COVID-19 pandemic.
2022 Cutting to lightweight, Saint Denis faced
Niklas Stolze on June 4, 2022, at
UFC Fight Night: Volkov vs. Rozenstruik. He won the bout after submitting Stolze via
rear-naked choke in the second
round. He was scheduled to face
Christos Giagos on September 3, 2022, at
UFC Fight Night 209, in
Paris. However, Giagos pulled out in August after a domestic accident. Consequently, Saint Denis faced Gabriel Miranda. He won the fight via
technical knockout in the early second round. This win earned him the
Performance of the Night award. However, he withdrew from the bout due to
ankle injury. Saint Denis was scheduled to face
Vinc Pichel on July 1, 2023, at
UFC on ESPN 48. However, Pichel pulled out in late-May due to injury and was replaced by Ismael Bonfim. Saint Denis won the fight via a first round rear-naked choke submission. For the beginning of his second contract with the UFC, Saint Denis faced
Thiago Moisés on September 2, 2023, at
UFC Fight Night 226, in
Paris. He won the fight by TKO at the second round. This fight earned him the
Fight of the Night bonus award. Saint Denis faced
Matt Frevola on November 11, 2023, at
UFC 295. He won the fight via knockout in the first round and the victory earned him the
Performance of the Night award. Frevola signed off with a salute to Saint Denis who responded in kind during this
veterans day. According to Dustin Poirier, the fight was initially pencilled at the start of the main card of
UFC 300 scheduled on April 13, 2024, at the
T-Mobile Arena in
Paradise, Nevada. After discussions, the fight was officially announced for UFC 299. Saint Denis lost the fight by knockout in the second round; however it earned him another
Fight of the Night award. Saint Denis faced
Renato Moicano on September 28, 2024, in the main event of
UFC Fight Night 243. He lost the fight by technical knockout due to doctor stoppage at the end of the second round as a result of damage done to his right eye. Following these defeats, Saint Denis chose a new headcoach, Nicolas Ott, who is also coaching
Nassourdine Imavov. However, Álvarez withdrew due to a hand injury the week before and was replaced by
Kyle Prepolec. He won the fight via an arm-triangle choke submission in the second round. Saint Denis faced
Maurício Ruffy in the co-main event on September 6, 2025 at
UFC Fight Night 258, in Paris. He won the fight via a face crank choke submission in the second round. This fight earned him another
Performance of the Night award. , April 2026 Saint Denis faced
Beneil Dariush on November 15, 2025, at
UFC 322. At the weigh-ins, Dariush weighed in at 157.2 pounds, 1.2 pounds over the lightweight non-title fight limit. The bout proceeded at catchweight and Dariush was fined 25 percent of his purse, which went to Saint Denis. He won the fight by knockout sixteen seconds into the first round. This fight earned him another
Performance of the Night award.
2026 Saint Denis faced
Dan Hooker in the co-main event on February 1, 2026 at
UFC 325. He won the fight by technical knockout in the second round. Saint Denis was offered to compete against
Michael Chandler on June 14, 2026 at
UFC Freedom 250 but he declined following an injury. He resumed training in April 2026 and assisted Spaniard
Joel Alvarez in preparation for his next fight. Saint Denis is scheduled to face
Paddy Pimblett on July 11, 2026, at
UFC 329. ==Professional grappling career==