At UFC 77 on October 20, Lesnar signed a one-fight contract to join the Ultimate Fighting Championship (UFC). On February 2, 2008, Lesnar made his first appearance in the UFC at UFC 81: Breaking Point, where he faced former UFC Heavyweight Champion Frank Mir. Due to his large hands, Lesnar wore 4XL gloves, becoming the second fighter in Nevada's combat sports history, following Choi Hong-man, to wear gloves of that size. Lesnar started strong with a takedown and landed multiple punches, but was penalized for hitting Mir on the back of the head. He continued to dominate with ground and pound, but Mir managed to secure a kneebar submission at 1:30 of the first round.
Despite his loss in his UFC debut, Lesnar received a new contract and was scheduled to fight Mark Coleman, a former UFC Heavyweight Champion and Hall of Famer, at UFC 82 on March 1. However, Coleman had to withdraw due to injury, and Heath Herring replaced him. In the fight, Lesnar knocked down Herring early in the first round and went on to dominate the fight, winning by unanimous decision with all three judges scoring the bout 30–26 in his favor.
Lesnar's next challenge came at UFC 91 on November 15, where he faced Randy Couture for the UFC Heavyweight Championship. Lesnar successfully defended Couture's takedown attempts and outperformed him in striking exchanges, eventually knocking him down and delivering ground and pound strikes until the fight was stopped, resulting in a technical knockout victory for Lesnar and the crowning of him as the new UFC Heavyweight Champion.
