Ricardo Arona

Soon after ADCC, Arona joined Fighting Network RINGS, a Japanese mixed martial arts promotion. His debut match would against aging sambo champion Andrei Kopylov, who Arona defeated by unanimous decision after controlling him positionally for most of the match. He then beat Jeremy Horn, and went to participate in the King of Kings tournament, but was eliminated at the first round by Fedor Emelianenko in Arona's first loss. Although the Brazilian controlled Emelianenko with takedowns and superior ground game, as RINGS's ruleset gave more scoring to aggression than positional control, Fedor was awarded the decision for cramming most of the submission attempts of the match.

Arona's next opponent was professional wrestler Hiromitsu Kanehara. Ricardo overwhelmed him with punches and takedowns, while Kanehara repeatedly reversed to top position and kept attacking, but Arona seized him at the second round with a kneebar and made him give in. Afterwards, Arona took part in a tournament for the RINGS Middleweight Championship, facing Jeremy Horn again in the first round and beating him again with a unanimous decision. The final match was against Gustavo Machado, who Arona defeated with kicks and punches in 1:29 to win the tournament and the title.

After being noticed in RINGS, Arona was offered a contract to join PRIDE Fighting Championships. He started his career there strong, defeating Pancrase veteran Guy Mezger, but his victory would be a controversial one. Mezger managed to stop Arona's takedowns and landed strikes, including a glancing head kick and several punches through the guard, which forced Arona to defend; it took two rounds for the Brazilian to come back, taking down Mezger and executing ground and pound until the end of the third round. Arona won a split decision, which was disputed by some sources, including the announcers of the match.[4]

Ricardo's next fight would be against another high ranked opponent, Dan Henderson. This time Arona was able to take down the wrestling champion, but Henderson executed his own takedown and landed knees and punches to the face until Arona could escape. The Brazilian switched his strategy and knocked down Henderson with a right hand, though the American still reversed and ended the first round attacking Arona's guard. At the second round, despite receiving a cut over his eye, Arona managed to overpower Henderson and punish him from the top. He looked to have secured a rear-naked choke in the final time, only for Henderson to escape, but Arona controlled him to end the match and got the split decision.

At PRIDE 23, Arona would go against Murilo Rua, a member of the Chute Boxe team, which was in a rivalry with Brazilian Top Team at the time. The two contenders fought a series of battles on the clinch, in which Arona imposed his superior wrestling, and almost finished the fight with a rear-naked choke and an Achilles lock at the end of the first round. The match went back and forth, with Murilo escaping from sparse submissions and landing his famed strikes, but Arona overpowered him with takedowns and kicks at the end and won the unanimous decision.

The victory over Rua was followed by Arona's first defeat in PRIDE, losing his fourth fight to Quinton Jackson in a spectacular knockout. Arona, outsized for the first time in the promotion, saw Jackson powering out of his holds, so he elected to stay in his guard to minimize damage. He seemed to knock out Jackson with an upkick, but as it wasn't so, he followed with a triangle choke, only for Jackson to suddenly lift him up and counter it with a powerbomb. The slam knocked Arona out and led the referee to stop the match, declaring Jackson the winner. After the bout, Arona protested and claimed that an illegal headbutt he received by Jackson at the end of the slam was what knocked him out; footage of the fight seems to support this,[5] but it was never officially acknowledged or investigated.

Four months later, Arona returned to PRIDE and won a fight against sambo champion Sergey Ignatov. On April 23, 2005, Arona entered PRIDE's Middleweight (205 lbs) Grand Prix. He won his first fight in the tournament against Dean Lister by unanimous decision, controlling the ADCC contender in a slow grappling contest.

In the second round of the Grand Prix, Arona won, in controversial fashion, his fight against the Japanese fighter Kazushi Sakuraba via corner stoppage. Although Sakuraba put Arona on his back and seemed to put to work his signature jumping stomps, Arona soon overwhelmed him, landing a series of brutal knees to the head, as well as hard soccer kicks to the face. The Brazilian's toenail accidentally slashed open the skin over Sakuraba's left eye, which the Brazilian capitalized on squeezing the cut and digging his finger into it in order to force the stoppage.[6] The Japanese tried to keep on, but Arona hit another flurry of knees to the head, and ended the fight landing soccer kick after soccer kick. He was declared the winner when Sakuraba's corner called a stop to the fight at the conclusion of the second round due to the amount of damage he had sustained.

His semifinal match, against rival Wanderlei Silva, was held on August 28, 2005. Arona surprised everyone when he defeated Silva, who was the PRIDE Middleweight Champion at the time, by decision, handing Silva his first middle weight defeat in PRIDE. Despite Silva's feared striking, Arona put him down with an early leg kick and worked on an extensive ground and pound, taking him down every time the fight got standing in order to continue with the punishment, which lasted until the last minute. After defeating Silva, Arona advanced to finals where he faced rising MMA fighter Maurício "Shogun" Rua, brother of Murilo Rua and Chute Boxe integrant. In a dominant performance, Rua reversed a takedown from Arona and got the advantage, opening his hips to catch Arona in an omoplata. Instead of submitting Arona, Rua used the advantage to get top position, then after a missed stomp, knocked Arona out with hammerfists to the face.[7]

On December 31, 2005 Ricardo Arona fought Wanderlei Silva for a second time, this time for the Middleweight Championship. Arona threatened to execute the same gameplan, but Silva countered it with a strong takedown defense and an opportunistic striking, including standing away from Arona's guard and kicking his legs. Arona got a decisive takedown at the second round and performed ground and pound, only for Silva to reverse and get free, which forced Ricardo to pull guard. At the final round, Arona managed to take Wanderlei to the ground and attack from top position, with Silva reversing him again and kicking at his legs. Arona lost the match in a discussed split decision.

On September 10, 2006 Arona faced Alistair Overeem. After withstanding the initial attack from Overeem, Arona landed an inside leg kick, injuring his opponent's leg and making him drop to the ground. Arona quickly overwhelmed Overeem on the ground by blanketing him and showed complete domination throughout the bout. He continued his assault until Overeem tapped out from a barrage of strikes.

Arona claims he suffered from a dengue hemorrhagic fever,[8] going into his fight against Sokoudjou on April 8, 2007 at PRIDE 34 and because of it, he was knocked out by Sokoudjou in just under 2 minutes.

After his loss to Sokoudjou, Arona's comeback fight against UFC veteran Marvin Eastman was canceled due to an injury that Eastman suffered during training. The fight with Eastman was rescheduled and took place at Bitetti Combat MMA 4 on September 12, 2009. Arona won the fight via unanimous decision.

In June 2014, Arona was offered a contract by Bellator MMA. However, he refused it and cited wanting to be one hundred percent physically healthy before recommitting to MMA as his reason.[9]