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.