A collapsible/expandable pet litter box includes a disposable compact box assembly having a base, a removable lid covering the base, and internally folded flaps connected to the base. The flaps are capable of being unfolded and extended upwardly from the base so as to substantially surround the base and cooperatively form, with the base, a box having sidewalls. One of the sidewalls forms a pet entry and exit aperture. Pet litter is encased within the base of the box. To construct the pet litter box, a first flap is unfolded into an upright position so that a side panel thereof forms a first lateral sidewall. Next, a second flap is unfolded into an upright position so that a side panel thereof forms a second lateral sidewall generally opposite the first lateral sidewall. Then rear panels of the first and second flaps are unfolded towards one another to form a rear sidewall. Front panels of the first and second flaps are unfolded towards one another to form a front sidewall and the pet entry and exit aperture. A barrier is removed to provide access to the pet litter. The sidewalls preferably include tabs which are forced into engagement with slits formed in the lid to securely hold the sidewalls in place.