With an area of 970 square kilometres (370 sq mi), it is the largest marsh on France's Atlantic coast and the second largest of the whole country, after the Camargue.. Extending across three departments (Vendée, Deux-Sèvres, and Charente-Maritime), it is situated west of Niort, north of La Rochelle, and south of Fontenay-le-Comte.
The Marais Poitevin was declared a Regional Natural Park (Parc naturel régional) in 1979, but it lost that status in 1997 as intensive agricultural development around the Marsh meant the unique character of the region was endangered, leaving only a core Interregional Park (Parc Interrégional du Marais poitevin) of 185 square kilometres (71 sq mi). Attempts to get back the full Park label started in 2002, leading to a new chart being proposed in 2006; accepted by the local authorities, it was rejected in late 2008 by the government due to a perceived "juridical fragility".
Tourism includes boating in traditional barques, which is a form of punting. There are several piers (embarcadères), from which boats can be hired. The myriad canals are covered in green duckweed (hence the Green Venice nickname) and the drained marsh land is home to a varied fauna.
The Marais Poitevin is also the most important area of angelica cultivation in France.