This depends on how you define the word ‘religion’. Usually, religion includes a belief in one or more gods. Using this understanding of the word, atheism is not a religion, as it is the lack of a belief in the existence of any gods.
The word ‘religion’ is difficult to define. Many of the belief systems commonly identified as religions have a lot in common. Christianity, Judaism, and Islam are all belief systems around the idea of a single, creator god. They also share many texts. Hinduism differs from these three – it is a system of many gods – but it is also commonly identified as a religion.
But what about Confucianism? That’s often called a religion, but it does not include an explicit belief in one or more gods. Does a belief system have to include a belief in one or more gods in order to be a religion, or is any belief system a religion? Some might argue that certain political philosophies, if held strongly enough, are also religions.
Most atheists are agnostic atheists (also called negative atheists or weak atheists). This means that they do not explicitly believe that god does not exist – they just do not accept that there is sufficient proof that god does exist. As such, this form of atheism is a lack of belief, and therefore cannot be a belief system, and cannot be a religion.