Réserve naturelle nationale du Bagnas, National nature reserve near Thau Basin, France.

The Réserve naturelle nationale du Bagnas covers 561.3 hectares between Agde and Marseillan, split by the Canal du Midi, the Bordeaux–Sète railway and the D 612 road. North of the canal lies the open lagoon Étang du Bagnas, with artificial fish ponds to the northwest; reed beds of Phragmites australis line the water, and temporary marshes, tamarisk groves and salt meadows lie to the south. A 400 m beach at the southern edge is used as a naturist beach.

The site was used for salt production for centuries before becoming a nature reserve in 1983. This transformation resulted from local efforts to prevent urban development projects in the area.

The name Bagnas derives from the Latin balneares, meaning baths, and probably marks the site of Roman thermal baths on the lagoon shore. Forms such as in ripam Bannars (1154) and stagni de Bagnars (1229) are recorded. The reserve is managed by ADENA, the Association de défense de l'environnement et de la nature des pays d'Agde.

Access to the reserve is free and self-guided, with numerous paths open on foot or by bicycle; vehicles are barred. The Maison de la Réserve, the ADENA management headquarters on the D 612 at Domaine de Grand Chavellt, holds an exhibition and information in French. The D 612, the Canal du Midi and the Bordeaux–Sète railway cut through the middle of the site.

The reserve's name derives from the Latin balneares, meaning baths, recorded as in ripam Bannars in 1154 and stagno de Banhars around 1186; it probably marks the site of Roman baths on the lagoon shore. The 400 m of beach at the southern edge belongs to the maritime public domain and is used as a naturist beach.