The 0.3-square-kilometre (74-acre) island lies along the Skagerrak coast, near the entrance to the Galtesundet, the main shipping channel leading to the town of Arendal on the mainland.

Graves from the Iron Age exist in two places on the island: Lakseberget and Støttavla.

Merdø is marked as an outport on every old Dutch maritime map since 1580, and it was one of the most important ports along the Skagerrak.

The island gradually depopulated completely, but the houses are still used as summer homes for vacationing families.

During the low season and all year round, both ferry companies offer taxi boat services.