 Rivera's first fight in the UFC was a unanimous decision victory over David Loiseau at "UFC 44".He then followed up with back-to-back submission losses to Lee Murray at "UFC 46" and Rich Franklin at "UFC 50" before rebounding with a unanimous decision win over Dennis Hallman at "UFC 55".Rivera was defeated by Chris Leben by technical knockout before appearing on The Ultimate Fighter 4 as a Middleweight contestant.