Assomption island is situated approximately 37km south of the eastern tip of Aldabra (9 43′ S, 46 31′ E).
This raised coral island has a mostly gently shelving rocky coastline, with the exception of the western coast that has an almost uninterrupted sandy beach that extends for approximately five kilometres. There is a large shelf extending out to sea on the eastern side of the island and a very steep drop close to shore on the western side.
Assomption is largely flat (see photo), and due to the devastating effect of guano mining which lasted until 1983, is dominated by expanses of bare rock and caves, and is sparsely covered with low. Two large sand dunes are prominent on the south eastern coast of the island, one of them reaching 32m high.
There is a concrete runway that runs from between the two sand dunes on the south east to the permanently manned settlement which is situated on the more sheltered western side of the island. The settlement is surrounded by Casuarina trees and there is an abandoned coconut palm plantation just south of it.