3 February 2017 Last updated at 17:45 GMT
Coral reefs are big underwater structures. They are made of coral, which is a bit like rock but is actually living, like trees and plants.
They're normally found in bright, clear water, like the Great Barrier Reef near Australia, so finding one in the muddy Amazon was a big surprise!
As well as being beautiful, reefs are very important, because different types of fish and underwater animals live there.
Scientists have been using mini-submarines to explore the reef, and they're hoping to find new species.
Greenpeace, who are a group who campaign to protect the environment, say that it's important to look after the reef.
Footage courtesy of Greenpeace.
A coral reef, larger than the size of London, has been discovered for the first time, in the Amazon river, in Brazil.