This work exposes the paleoenvironmental evolution of the Belle-Henriette lagoon and suggests a hypothesis about the infilling of the former Golfe des Pictons, in western France. Nowadays known as the Marais poitevin, this marsh is the second biggest wetland in France, covering nearly 100,000 ha. A transect composed of five sediment cores was collected in 2019 using both a Beeker gravity corer and a vibracore corer, starting from the sandy barrier to the center of the lagoon. High-resolution elemental analyses of sediment cores were done using an Avaatech XRF core scanner, and X-radiographs were taken using a Scopix system. Grain size analyses were measured at a resolution of 1 cm using a Malvern 2600 laser diffraction particle size analyzer. Dating was derived from sedimentary profiles of 137Cs and 210Pb, measured on an aliquot of 2 g dried sediment using a low background, welltype γ spectrometer (Canberra). Historical maps were extracted from the Bibliothèque François-Mitterrand and from departmental archives. Nearly fifty maps were studied to understand the composition of each detected layer and to reconstruct the historical landscape of the western part of the Marais poitevin. This work gives clues about the creation of this marsh during the last few centuries, by the infilling of the former Golfe des Pictons and the evolution of the Lay river (former St. Benoist) estuary. We also expose the creation of the Belle-Henriette lagoon during the last decade by the progradation of several coastal spits.