Tromøy is a former municipality in the old Aust-Agder county in Norway. The 30-square-kilometre (12 sq mi) municipality existed from 1878 until its dissolution in 1992. The municipal area is now part of the municipality of Arendal in Agder county. The administrative centre was the village of Brekka, where the historic Tromøy Church is located. The municipality encompassed all of the island of Tromøya as well as many smaller surrounding islands such as Merdø, Gjesøya, Skilsøy, and Tromlingene.

Tromøya or Tromøy is the largest island in Southern Norway. The 28.6-square-kilometre (11.0 sq mi) island is entirely located in the municipality of Arendal in Agder county, Norway. The island has about 5,300 residents which gives it a population density of about 185 inhabitants per square kilometre (480/sq mi). The island is located directly across the harbor from the town of Arendal. The highest point on the island is the 95-metre (312 ft) tall Vardåsen. The island is separated from the mainland to the north by the Tromøysundet strait and it is separated from the island of Hisøya to the southwest by the Galtesundet strait.