While the lack of belief in a deity or deities cannot in itself be a standalone religion, Mike is right in pointing out that holding such a belief does not prevent one being defined as "religious"
per se. In its strictest form, Buddhism is an atheistic (or, if you prefer, nontheistic) belief system, given that the rejection of deities on grounds of falsehood was deemed a significant step in Buddha's own enlightenment. Yet few outside of academia would deem Buddhism not to be a religion, even if in technical terms it's a tricky one. Likewise, the term "cult" is often applied to groups or movements with no evident religious motivations despite its actual definition - in one debate a while back, Toz raised the example of the "cult of Stalin", which at its heart was by definition atheistic since Stalin was himself an atheist, yet also displayed traits mimicking religious practice. So while atheism is not on its own a religion, it is possible for a set of atheistic philosophies to take on quasi-religious form nonetheless.