North-western Arabia is marked by thousands of prehistoric stone structures. Of these, the monumental, rectilinear type known as mustatils has received only limited attention. Recent fieldwork in AlUla and Khaybar Counties, Saudi Arabia, demonstrates that these monuments are architecturally more complex than previously supposed, featuring chambers, entranceways and orthostats. These structures can now be interpreted as ritual installations dating back to the late sixth millennium BC, with recent excavations revealing the earliest evidence for a cattle cult in the Arabian Peninsula. As such, mustatils are amongst the earliest stone monuments in Arabia and globally one of the oldest monumental building traditions yet identified.