Tromøy (historic: Tromø) is a former municipality in the old Aust-Agder county in Norway. The 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.
The municipality had two churches: the centuries-old Tromøy Church and the relatively new Færvik Church. Some of the main villages in Tromøy were Færvik, Brekka, Brattekleiv, Sandnes, Revesand, and Pusnes. The municipality was connected to the mainland by the Tromøy Bridge, the only road connection to Tromøya. The other islands of Tromøy were only accessible by boat.
The municipality of Tromøy was established on 1 May 1878 when the municipality of Østre Moland was divided into three municipalities: Tromøy (population: 2,320), Barbu (population: 4,874), and Østre Moland (population: 2,524). On 1 January 1992 there was a major municipal merger where the following municipalities were merged into the municipality of Arendal: Tromøy (population: 4,711), Moland (population: 8,148), Øyestad (population: 8,679), Hisøy (population: 4,026), and the town of Arendal (population: 12,478).
The municipality (originally the parish) is named after the island of Tromøya (Þruma). The first element is likely derived from the word þrǫmr which means "rim", "edge", or "border". The last element is øy which means "island". Historically, the name was spelled with the suffix -ø which is the Danish spelling for "island". During the 20th century, the spelling was changed to -øy, using the Norwegian spelling for "island".
The coat of arms was granted on 23 August 1985 and in use until the municipal merger on 1 January 1992. The official blazon is "Per fess nebuly argent and azure" (Delt av sølv og blått ved skysnitt).