Following the loss, "Ninja," the older brother of Mauricio "Shogun" Rua, announced his retirement from MMA on his Twitter account.
"I would like to announce officially my retirement from pro MMA fights," Rua wrote. "It was an amazing run, and [it's] a very tough moment, but there comes a time for all in life, and it's time to move on. I am proud of all I did in MMA and all experiences I had. I will continue to work with MMA, doing seminars, [teaching] classes, training fighters, and doing my share to help our sport that I love so much. It's time now to help others and enjoy my family, my wife, my kids and move [on]."
The 31-year-old Rua leaves MMA with a 20-12-1 record. He began his career in May 2000, and along the way, fought for PRIDE, DREAM, EliteXC and Cage Rage.
The win improved Watson's record to 14-4 overall at 10-1 in his last 11 fights.
In other BAMMA 6 action, Frank Trigg defeated John Phillips via first-round TKO and Matt Ewin beat Ivan Salaverry via unanimous decision.
Full results from the event, which was held at Wembley Arena in London, England, are below, courtesy of MMA Mania:
Tom Watson def. Murilo Rua via knockout in round three
Frank Trigg def. John Phillips via TKO (doctor stoppage for cuts) in round 1
Matt Ewin def. Ivan Salaverry via unanimous decision
Leonardo Santos def. Jason Ball via unanimous decision
Che Mills def. Marcio Cesar via KO in round 1
Scott Jansen def. Craig Chesters via TKO in round 2
Catalin Zmarandescu def. Sharif Mohammed Ali via submission in round 1
Valentino Petrescu def. Joao Paulo de Souza via majority decision
Shane Omer def. Aaron Wilkinson via TKO
Sam Watling def. Leigh Cohoon via unanimous decision
Costas Doru and Jeremy Petley fought to a draw
Lee Taylor def. Kenny Moyston via TKO
Ryan White def. Fraser Opie via submission
Murray Fullerton def. Charles Scott via submission