Isthmus Bay is a body of water in South Eastern Tasmania.
It is a bay of Bruny Island Isthmus Bay was discovered on 1 May 1792 by M. de Cretin, of Antoine Bruni d'Entrecasteaux's expedition along the eastern coast of Australia.
On 21 May M. de St. Aignon, while searching for an exit from the channel, beached his boat in Isthmus bay, and waded ashore naked holding his gun and his compass.
On 13 February 1793, M. de Well and the geographer M. Beaupre, returned to Isthmus Bay.
They named the isthmus "St. Aignon" after M. de St.