Hisøy (historically: Hisø) is a former municipality in the old Aust-Agder county in Norway. The municipality existed from 1881 until 1992 when it was merged into the municipality of Arendal in what is now Agder county. At the time of its dissolution, it was the smallest municipality in the whole county. The municipality encompassed the islands of Hisøya, Gjervoldsøy, Havsøya, Ærøya, Store Torungen, Lille Torungen, and several other small, unpopulated islands. The administrative centre was the village of Kolbjørnsvik on Hisøya island.
Hisøy Church was the main church for the municipality, located in the village of His in the central part of Hisøya.
The municipality of Hisø was created on 1 January 1881 when the municipality of Øyestad was split into Øyestad (population: 4,474) and Hisø (population: 2,652). On 1 January 1992, there was a major municipal merger involving Hisøy. The municipalities of Hisøy (population: 4,026), Tromøy (population: 4,711), Øyestad (population: 8,679), Moland (population: 8,148), and the town of Arendal (population: 12,478) all merged, forming a new, much larger municipality of Arendal.
The municipality (originally the parish) is named Hisøy (or historically Hisø) after the old His farm (Hís) since the first Hisøy Church was built there. The meaning of the first element of the name is uncertain. It could come from the word hís which means "the cut" (possibly referring to how the river Nidelva turns near the island). Another The last element is øy which means "island". Historically, the name was spelled Hisø with the Danish spelling of "island" (ø), but during the early 20th century, the spelling was "Norwegianized" by using the Norwegian spelling of "island" (øy).
The coat of arms for Hisøy municipality was granted on 12 December 1986 and it was in use until 31 December 1991 when the municipality ceased to exist. The official blazon is "Gules, two lighthouses argent issuant from the base" (I rødt to oppvoksende sølv fyrtårn). This means the arms have a red field (background) and the charge is two twin lighthouses.