Located in the southern Subalpine Chains, the Dévoluy-Bochaine (D-B) area displays an “ante-Senonian” (Turonian) unconformity classically interpreted as the result of a compressional event. We argue that the D-B unconformity results from a major synsedimentary collapse on the slope of the European paleomargin. During the Tithonian-Late Cretaceous interval, the D-B area was located on a steep slope between the European platform (Vercors) and the Vocontian basin of SE France where mass-flows and resedimented deposits accumulated. Under the unconformity, the Dévoluy folding affects the Kimmeridgian-Cenomanian series detached on the Mid-Upper Jurassic marl-rich “Terres Noires”. Truncation of the top of the collapsed unit resulted from normal faulting and fragmentation of the poorly consolidated sediments and from their erosion by deep sea currents. Neither subaerial deposits nor shallow water conglomerates occur at the unconformity interface, where fault scarp breccias are locally exposed. The subsequent, mostly Coniacian-Maastrichtian onlapping formation (“Calcaires en Lauzes”) consists of channelized sandy calci-turbidites where slumping events occur again. To conclude, the Dévoluy-Bochaine massifs offer the first example exposed in the External Western Alps of a major synsedimentary collapse comparable to the Niger or Israel offshore study cases.