A modular animal pen system comprises front and rear panels and opposed side panels extending between the front and rear panels to form a primary enclosure. The front panel has adjacent doorway apertures formed therein to receive respective selectively closable gates, and a mullion is disposed between the doorway apertures. The mullion has a first slot, and a second slot in the rear panel is in registration with the first slot. The animal pen system further comprises a removable divider panel whose longitudinal ends are sized to fit removably within the respective slots so that the divider panel can divide the primary enclosure into two secondary enclosures. The side and divider panels may be formed by interchangeable profile panels, and the front and rear panels may be formed by interchangeable face panels connectable end-to-end to form a junction slot therebetween sized to receive the longitudinal ends of the profile panels.
