A self-consistent regional-scale model of the crust and upper mantle for the southern Arabian Shield and Red Sea paar has been constructed from an integrated interpretation of seismic deep refraction, regional gravity, aeromagnetic, heat flow, and surface geologic data. The Shield consists of two 20-km-thick layers of crust with an average compressional wave velocity in the upper crust of about 6.3 km/s and in the lower crust of about 7.0 km/s. This crust thins abruptly to less than 20 km near the southwestern end of the transect, where Precambrian outcrops about the Cenozoic rocks, and to 8 km beneath the Farasan islands. The data over the Red Sea westward of Precambrian outcrop are fit satisfactorily by an oceanic crustal model. The major velocity discontinuities occur at about the same depth across the entire shield and indicate horizontal metamorphic stratification of the Precambrian crust. Several lateral inhomogeneities in bulk physical properties have been identified in both the upper and lower crust of the shield, indicating bulk compositional variations. The subcrustal portion of the model is composed of a hot, low-density lithosphere and asthenosphere beneath the Red Sea which is systematically cooler and denser to the northeast. This model provides a mechanism that explains the observed topographic uplift, regional gravity anomaly pattern, heat flow, and mantle seismic velocities. Such a lithosphere could be produced by upwelling of hot asthenosphere beneath the Red Sea which then flows laterally beneath the lithosphere of the Arabian plate.