Southworth's first major exposure in the mixed martial arts world was when he faced Vitor Belfort at PRIDE 13. Southworth was overwhelmed by the Brazilian veteran and lost by rear naked choke in the first round. Southworth then appeared on Season 1 of The Ultimate Fighter. He won his first preliminary fight against Lodune Sincaid, but lost the second versus Stephan Bonnar by split decision. Bobby returned to the house as a possible replacement for Forrest Griffin, should he not have been medically cleared to fight in the semifinals, but Forrest's cut healed safely and Southworth did not fight again until the undercard of the finale, where he fought a man he picked on in the house Sam Hoger and was defeated. It was a unanimous decision victory for Sam Hoger. Southworth next moved to Strikeforce, where his first fight was against James "The Sandman" Irvin. The bout was declared a No-Contest after a mere 17 seconds when in a freak accident, Southworth and Irvin's clinch pressed against the cage, pushing open the cage door and causing Irvin to fall out of the ring. In his next fight, Southworth was given the opportunity to fight for Strikeforce's vacant light heavyweight championship against Vernon "Tiger" White. Southworth won a decision victory, taking White down repeatedly and controlling the pace of the match. Southworth's next fight was a non-title bout against Anthony Ruiz, a fight that Ruiz won by way of TKO in round 2. A rematch was set up, but this time it would be for Southworth's championship. In a rather slow-paced fight, Southworth defeated Ruiz via 5-round Unanimous Decision on June 27, 2008 and thus became the only man to successfully defend the championship. In his next title defense, at on November 21, 2008, Southworth lost the light heavyweight championship against former UFC Light Heavyweight fighter Renato Sobral "Babalu" by TKO of the 1st round at 5:00. Southworth was winning the round, having scored a takedown and controlling the clinch, until "Babalu" opened up a horrendous gash above Bobby's left eye with ten secondsto go. "Big" John McCarthy stopped the action to let doctors inspect, and Southworth finished the round by dropping "Babalu" with a punch just before the bell. However, when doctors got a better look at the wound between rounds, the fight was stopped due to the size and depth of the cut. Southworth announced he would like a rematch and Sobral verbally agreed, however a rematch was never organized and following Sobral's 2013 retirement it is unlikely a rematch would ever occur. Southworth also tried out for 11th season of The Ultimate Fighter. However, despite the fact that the both Middleweights and Light-Heavyweights were invited to try out, the UFC decided that the season would focus exclusively on Middleweights.
Personal life
Bobby also said on an episode of The Ultimate Fighter that he's adopted. Bobby has four children, 3 daughters and one son.
Championships and accomplishments
Strikeforce
*Strikeforce Light Heavyweight Championship
*One Successful Title Defense
*The only fighter to have successfully defended the Light Heavyweight Championship
