Bradley dominated the fight, scoring three knockdowns in the early rounds of their supporting main event at the MGM Grand Garden Arena.
Casamayor's corner threw in the towel to signal the end of the fight as the Cuban fighter had too much of Bradley's power assault.
Bradley retained his WBO welterweight champion and boost his chances of facing pound for pound king Manny Pacquiao with his impressive performance.
The fight is crucial to Timothy Bradley in his bid to land a lucrative fight with Pacquiao, who has just been called by Mayweather's team for a mega fight at the MGM Grand in May next year.
Bradley has just signed a fight contract with Bob Arum's Top Rank Promotions after a lingering legal battle with his former promoters. Top Rank is the same company that promotes Manny Pacquiao fights.
After signing up with Bob Arum's Top Rank Promotions, Bradley said he hopes to face Pacquiao or Mayweather, saying he has earned the right to face these guys.
"I am looking to fight best pound for pound fighters in the game. That's Manny Pacquiao or Floyd Mayweather. I have already paid my dues. I am tree time world champion. Let me get a shot. I feel that I've earned the right to fight these guys. I am going to continue to win every fight that I possibly get in so that I can get into contention to fight these guys."
Bradley has not fought for almost a year since winning a technical decision over Devon Alexander last January 29, also at the MGM Grand in Las Vegas, Nev.
With his win over Casamayor, Bradley has improved his professional record ro 28 wins without a single defeat.