On April 5th 2007, a major collapse of the Dolomieu summit crater occurred at Piton de La Fournaise volcano (La Réunion Island). To constrain the origin and the dynamics of the collapse, we modelled continuous GPS displacements preceding and accompanying this event. Our results reveal that the Dolomieu collapse formed by a “piston-like” mechanism, in which the fast draining of the shallow magma reservoir at the beginning of the March 30th–May 1st 2007 eruption caused a gravity-driven downward displacement of its roof. During the five days preceding the collapse, summit deflation was caused by volume change through the fractured rock column above the shallow magma chamber due to the closure of voids or small-scale cavities or the upward stoping and migration of an underground cavity. Since 2000, weakening of the rock column by repeated refilling and draining of the shallow magma chamber favoured its destabilization. On April 5th, 7 h before the onset of the collapse, the deflation source was located at ~ 300 m depth revealing the initiation of the collapse at very shallow depth. The sudden collapse on April 5th was followed by continuous cyclic subsidence until April 6th. Each cycle was characterized by a progressive inward deflation of the summit zone and ended by sudden summit outward displacements caused by stress relaxation following a collapse event. Subsidence of the stoped column in the magma reservoir during the collapse of the crater on April 5th and 6th acted as a piston thus increasing the eruption rate. The collapse occurred in only 24 h by successive events and increased the depth of the Dolomieu crater by 340 m. After April 6th, only minor readjustments of the summit cone occurred with slight summit deflation and small landslides of the caldera walls.