evidence: Although Middle Palaeolithic stratified and dated sites are still rare in Arabia, recent archaeological, palaeoenvironmental, population genetic, geomatic and geochronological studies have noticeably contributed to a re-evaluation of the prehistory of the region. Here, we report the discovery of a stratified open-air Middle Palaeolithic site in central Saudi Arabia, a novelty given the paucity of dated Pleistocene lithic assemblages in the region. The site of Umm al-Sha’al is located in the Rufa Graben where a substantial number of Middle Palaeolithic surface occurrences have been reported. It contains artefacts produced using Levallois technology, indicative of Middle Palaeolithic human exploitation of locally abundant quartzite raw material. The site comprises two horizons with archaeological finds dating to Marine Isotope Stages 5 and likely 7 or even older. During this period, hunter-gatherers would have benefited from bodies of water and streams in the vicinity of the site, which were active during humid phases, contributing to the development of a significant biomass. Our discovery supports the hypotheses of an early colonisation of inland Arabia by hominins and of a solid link with North and East African lithic traditions of the Middle Stone Age, which awaits further refinement. [...] Vaslet D, Al-Muallem MS, Maddeh SS, Brosse JM, Fourniquet J, Breton JP, Le Nindre YM (1991) Explanatory notes to the geologic map of the ArRiyad Quadrangle, Sheet 24 I, Kingdom of Saudi Arabia. Saudi Arabian Deputy Ministry for Mineral Resources, Jeddah. Geosciences Map 121, 1–54