The first name of the Lac de Longemer dates from an 11th-century manuscript that described the Longum Mare when mentioning the locality of the hermit Bilon's cell. The meaning Long Sea refers to the elongated shape of the lake and its designation by the term sea in the local dialect. From 1756, the lake took on many names (Longe Mer, Longemeix, Langesehe, Langsee, Langen See, Longue Mer...) depending on who controlled the region, until it obtained its current denomination.