Tverrdalsøya is an island in Arendal municipality in Agder county, Norway.

The 3.4-square-kilometre (1.3 sq mi) island lies between the Oksefjorden, Eikelandsfjorden, and Kilsundet straits just off the mainland coast at the entrance to the Tvedestrandsfjorden which leads north to the town of Tvedestrand.

The island of Flostaøya lies immediately to the south and the island of Borøy lies immediately to the east.

[1] Historically, the island was a part of the municipality of Dypvåg until 1902 when it was made a part of the new municipality of Flosta.

[2] This article about an island in Agder is a stub.