Edmagno Maguinho claims his first bite tasted strange and felt like something was moving around in his mouth, but managers claimed the lunch had been thoroughly cooked on the grill.
A Burger King customer filmed the horrifying moment he claims he discovered his burger was crawling with maggots.
Dozens of larvae are seen swarming all over the snack which Edmagno Silva Maguinho says was sold to him on Sunday.
Mr Maguinho, who posted the video online, was out with his wife when they decided to buy two cheese burgers at a branch of the fast food chain in Brazil.
He alleges that it was after the very first bite that he noticed a strange taste, and he felt something moving around in his mouth.
He spat out the mouthful out and claims he made a stomach-churning discovery.