This article identifies a military prison (carcer castrensis) in the Roman legionary fortress at Lambaesis (Tazoult, Algeria) and contextualizes the space among North African carceral practices evidenced in epigraphic, papyrological, and literary sources of the first through fourth centuries CE. The identification is made on the basis of architectural comparanda and previously unnoticed inscriptional evidence which demonstrate that the space under the Sanctuary of the Standards in the principia was both built as a prison and used that way in antiquity. The broader discussion highlights the ubiquity of carceral spaces and practices in the ancient and late ancient Mediterranean, and elucidates some of the underlying practices and ideologies of ancient incarceration.