Abstract The southern Rhine Graben was investigated by the seismic reflection ecors-dekorp program. Gravity data available in this region have been reprocessed in order to develop a coherent map along the seismic line. The new map has been corrected for the gravity effects of tertiary sediment and crust-mantle discontinuity. The strong negative gradient towards the Alps may be explained by a density contrast of 300–500 kg/m 3 at the crust-mantle interface. The residual map shows a negative, long-wavelength anomaly superimposed on the Moldanubian domain of the Variscan belt, along a Vosges-Black Forest-Swabian Jura axis. This anomaly cannot be explained by the properties of the Variscan crust but may result from a low-density body within the upper mantle. However, its ENE-WSW direction rules out any connection with the formation of the Tertiary grabens, which trend north-south. The recent uplift of topography and the volcanic activity (< 20 Ma) of the southern Rhine Graben may partly be a consequence of an asthenospheric upwelling beneath this area. The analysis of the residual short-wavelength anomalies shows a fundamental difference between the light Variscan terranes of the Moldanubian zone and the dense “Barrandian” Saverne-Kraighau structures of the Saxothuringian domain. A density cross-section along the ecors-dekorp line confirms that the Moho uplift extends beyond the Graben, whereas thinning of the lower crust affects the Graben area only. We interpret this crustal structure as an initially thin Hercynian crust, later deformed by asymmetric extension during Oligocene rifting.