Ermentar of Noirmoutier, also called Ermentarius Tornusiensis (died mid-860s), was a monk and historian of the abbey of Saint-Philibert de Tournus.
He wrote a vivid prose chronicle, De translationibus et miraculis sancti Filiberti, recounting the disruption of his community by Viking raids and its transfer from the Breton island of Noirmoutier to several locations in France before it was finally settled at Tournus in 875 (after his death).
The monks had begun construction on a new monastery in the villa of Déas under Hilbod's predecessor, Arnoul, who died in 824/5.
It was there around 839 that Ermentar wrote his first work, the Vita sancti Filiberti, a life of Saint Philibert based on an earlier seventh-century biography.
In 836, they finally relocated inland permanently and in 843 the Vikings took over Noirmoutier and made it their base for a series of raids into France.