Regional Bouguer gravity anomalies in southern New Mexico have been analyzed by two-dimensional wave number filtering and polynomial trend surface analysis of the observed gravity field. A prominent, regional oval-shaped positive gravity anomaly was found to be associated with the southern Rio Grande rift. Computer modeling of three regional gravity profiles suggests that this anomaly is due to crustal thinning beneath the southern Rio Grande rift. These models indicate a 25 to 26-km minimum crustal thickness within the rift and suggest that the rift is underlain by a broad zone of anomalously low-density upper mantle. The southern terminus of the anomalous zone is approximately 50 km southwest of El Paso, Texas. A thinning of the rifted crust of 2-3 km relative to the adjacent Basin and Range province indicates an extension of ~9% during the formation of the modern southern Rio Grande rift. This extension estimate is consistent with estimates from other data sources. We interpret the crustal thinning and anomalous mantle to result from magmatic activity related to surface volcanism and high heat flow in this area.