Depends on context. In regards to a lot of Federal discrimination legislation like that in Canada, atheism is actively protected under the subheading of "religious belief".

In some Provincial discrimination legislation it is covered by the term "creed" rather than religion but if something goes to the level of the Federal Supreme Court then for purposes of protection against discrimination or for matters of hate speech atheism is considered for all purposes legally a religious belief.

Linguistically laws often term Atheism more broadly as a set of spiritual beliefs. Believing entirely in the absence of the supernatural is still describing what you believe about spiritual matters. Atheism is not a complete absence of belief. You still have things you believe about spirits - you believe they do not exist. If you truly had no spiritual beliefs you would have to be completely unaware that the concept of the belief of the spiritual exists at all including disbelief in the spiritual.

Fundamentally law that applies atheism as a religion employs this logic despite religion having connotation of an internal dogma - something absent in atheism.