Ronaldo Souza is a Brazilian mixed martial artist, submission grappler and Brazilian Jiu-Jitsu practitioner. He is one of the top grapplers in the world, being the 2004 and 2005 Openweight World Jiu-Jitsu Champion and 2005 ADCC Absolute Division runner-up, only losing to Roger Gracie. His nickname Jacaré is Portuguese for the species of alligatoridae living in his town of birth.Ronaldo made his MMA debut on September 9, 2003 at Jungle Fight's inaugural event where he lost to Jorge "Macaco" Patino by KO at 3:13 into the first round.

He returned to MMA eight months later at Jungle Fight 2 securing a submission win over Victor Babkir under a minute into the first round. His next fight was in April 2006 at Jungle Fight 6 where he got his second win, defeating by Alexander Shlemenkoan by an arm triangle choke submission. He went on to fight twice before the end of 2006, winning both fight by submission during the first round.

At the end of 2006, Jacaré fought and drew with former UFC Heavyweight Champion Randy Couture in a grappling contest. After the match he was invited by Couture to train at his gym. Jacaré accepted and now trains at Xtreme Couture with the likes of Forrest Griffin, Tyson Griffin, Martin Kampmann, Wanderlei Silva and Randy Couture.

Ronaldo's first fight of 2007 was on May 19 with him scoring a submission win due to strike over Bill Vicick at Gracie Fighting Championships: Evolution. He would next fight twice within a two week period on September 29 and October 13, scoring two submission wins both within the first round.

In 2008 Jacaré signed with newly created Japanese MMA-promotion DREAM, and was scheduled to participate in the 2008 DREAM Middleweight Grand-Prix.

In the first round of the tournament, at DREAM.2 on April 29, 2008, he defeated Ian Murphy by a rear naked choke submission in the first round. This secured him a place in the quarterfinals which took place at DREAM.4 on June 15 were he fought former ICON Sport Middleweight Champion Jason  Miller "Mayhem". In the fight Jacaré continuously went for submissions, but Miller showed good submission-defense escaping multiple rear naked choke, leglock and armbar-attempts and mounted his own offense on the feet. Jacaré was awarded an unanimous decision victory which secured him a place in the semi-final of the Middleweight Grand-Prix.

In the semi-final of the tournament, which took place at DREAM.6 on September 23, Jacare defeated Zelg Galesic early in the first round by way of an armbar submission. This victory earned him a place in the finals of the DREAM Middleweight Tournament and a shot at the DREAM Middleweight Championship against Gegard Mousasi at the end of the night. In the finals of the tournament, Jacare was knocked out early in the first round by way of an upkick as Jacare attempted to dive past Mousasi's guard to land a strike of his own. This loss placed Jacare second overall in the DREAM Middleweight tournament.