If you take the definition of religion as:
re·li·gion Pronunciation Key (r-ljn)
n.

1.
  • 1. Belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe.
    2. A personal or institutionalized system grounded in such belief and worship.

2. The life or condition of a person in a religious order.

And therefore acknowledge there is no supernatural power nor 'institutionalized system' nor worship required in Buddhism, it is a philosophy.

However, if you take the definition of 'religion' as:
3. A set of beliefs, values, and practices based on the teachings of a spiritual leader.
4. A cause, principle, or activity pursued with zeal or conscientious devotion.

The argument that it is a religion can then be made as it is based upon the teachings of one Siddhartha Gautama among others. Though he told all who listened to question anything, even if he said it, and not to follow unless in complete accordance with ones own logical thinking. Thus the cause or principle may be pursued zealously but is differentiated by the individual and not set in stone or dictated by any work of literature.
For this reason I have concluded it to be a philosophy, by anothers reasoning it could be called a religion; whatever.