UFC 61: Bitter Rivals was a mixed martial arts event held by the Ultimate Fighting Championship on Saturday, July 8, 2006. The event took place at the Mandalay Bay Events Center, in Las Vegas, Nevada and was broadcast live on pay-per-view in the United States.
The main event was a Heavyweight Championship match between champion Tim Sylvia and former champion Andrei Arlovski. This was the third time these fighters had met, they had split the first two meetings. The co-main event was the highly anticipated re-match of TUF 3 coaches Tito Ortiz and Ken Shamrock.
It was announced during the event that Chuck Liddell would fight PRIDE star Wanderlei Silva in November if Liddell won his upcoming match against Renato "Babalu" Sobral at UFC 62. Negotiations between the UFC and PRIDE failed however and the Liddell-Silva fight never occurred in November 2006. The two would eventually meet in December 2007 at UFC 79.
The number of pay-per-view buys for UFC 61 in the United States was approximately 775,000, which generated gross PPV revenue of approximately $30,960,000. The disclosed fighter payroll for the event was $676,000. With a live gate at $3,350,775, it nearly eclipsed the all-time mark for UFC events set at UFC 57.