Thales Leites Lourenco , or simply Thales Leites (born September 6, 1981 in Rio de Janeiro, Brazil), is a Brazilian mixed martial artist, currently fighting at middleweight for the Ultimate Fighting Championship. Leites made his debut with the organization on November 11, 2006, after a bout originally planned for October fell through due to visa-processing issues. A relative newcomer to the sport, Leites made his mixed martial arts debut at Shooto Brazil in 2003, defeating Felipe Arinelli by arm-triangle choke submission. Leites would win his next two fights in Vitória before making his debut in the United States in Hawaii's Rumble on the Rock promotion in 2005, where he defeated Adam Roland in his fourth consecutive mixed martial arts victory.

After a successful outing in the U.S., Leites returned to Brazil to fight Gustavo Machado, who was coming off two wins in the Japanese Pancrase organization. Leites defeated the more experienced Machado by submission in the third round, and was invited back to Rumble on the Rock to fight Hawaiian Ronald Jhun in September. Despite fighting a hometown favorite, Leites notched his sixth victory, defeating Jhun by technical knockout in the later half of the fight.

Although Leites held an unblemished record, he didn't gain much mainstream exposure until his bouts with Osami Shibuya in Japan's MARS promotion, and Jose Landi-Jons in Brazil's Jungle Fight promotion. In both fights, Leites was much less experienced than his opponents; about Landi-Jons, Leites admitted, "He was already famous when I was not even interested" in mixed martial arts. Despite the experience disparity, Leites defeated both opponents, scoring a unanimous decision over Shibuya, and submitting Landi-Jons with an arm-triangle choke, increasing Leites' professional record to 9 wins, 0 losses.

In September, it was announced that Leites would make his UFC debut against veteran middleweight Nathan Marquardt at Ortiz vs. Shamrock 3: The Final Chapter. However, due to issues with his travel documentation, Leites was not able to compete at the event as planned.

Leites made his debut at The Ultimate Fighter 4 Finale, where he lost his first professional fight by unanimous decision to Danish kickboxer Martin Kampmann.

In his next fight, at UFC 69, he defeated The Ultimate Fighter 4 contestant Pete Sell. Leites next earned a submission victory over Floyd Sword at The Ultimate Fighter 5 Finale by arm-triangle choke. He was then scheduled to fight Ryan Jensen at UFC 74. Leites accepted the fight after Travis Lutter, Jensen's original opponent, was forced to withdraw from the event. Leites defeated Jensen by armbar submission in the first round, raising his UFC record to 3 wins, 1 loss.

Leites was scheduled to fight Nathan Marquardt at UFC 81, but withdrew from the bout after breaking his hand in training. Leites fought Marquardt at UFC 85, winning by a controversial split decision due to point deductions in the 2nd and 3rd rounds to Marquardt.

On the undercard of UFC 90 Leites was able to earn a rear naked choke submission victory over dangerous striker Drew McFedries.