This question was (sort of) answered in the past in this thread.

Sometimes the Pirahã people are given as an example of an atheist society. They have probably been living their mode of life since their arrival in the Amazon, though it's difficult to tell, and the reason why will become clear in a moment. If by 'atheist' you simply mean 'without gods', then sure, they're atheist.

Daniel Everett spent years studying Pirahã culture and language. From his book Don't Sleep, There Are Snakes:

...if all Pirahã myths must exemplify immediacy of experience, then the scriptures of many world religions, such as the Bible, the Koran, the Vedas, and so on, could not be translated or discussed among the Pirahãs, because they involve stories for which there is no living eyewitness. This is the main reason that no missionary for nearly three hundred years has had any impact on the Pirahãs' religion. (Everett 142)

They don't care much about things that you haven't directly seen, so they do not care about 'gods' as Eurasians know them. They also don't have oral histories.

However, they have a strong belief in a concept Everett translates as 'spirits'. He describes

...episodes in which the Pirahãs doubted that I was the same person and expressed their belief that other white people were spirits, changing forms at will... (141)

They seem to see spirits physically, and even seem to have names for them. One is "Xigagaí, one of the beings that lives above the clouds" (xvi), whom Everett couldn't see, while his companion Kóhoi insisted he was standing on the beach. Everett describes their belief:

Pirahãs see spirits in their mind, literally. They talk to spirits, literally. Whatever anyone else might think of these claims, all Pirahãs will say that they experience spirits. (141)

The Pirahã are godless, but they believe in the explicitly supernatural phenomenon of shape-changing spirits, which does not fit with the modern uber-scientific notion of atheism today. This is about as close as it gets.