The enormous reptile weighing 400kg was found by construction workers following an explosion in a cave in Altamira, Pará.
Footage filmed by one of the workers shows the snake’s gigantic body, which measures one metre in diameter.
One user wrote: ‘I’m not sure this is real, but if it is, shame on these people for killing it! A snake that big has probably been alive for a very long time now, just to be killed for a few pictures?? What a waste…’
The Guinness World Record for the longest snake in captivity is currently held by Medusa, a snake in Kansas City, US, which measures 25 ft 2in long.
The Belo Monte Dam is a hydroelectric dam complex under construction on the Xingu River.
It is set to be the fourth-largest dam installation in the world behind the Three Gorges Dam in China.