Gjerstad is a municipality in Aust-Agder county, Norway. It is located in the traditional region of Agder, in the southeastern part of the county, along the border with Telemark county.The parish of Gjerstad was established as a municipality on 1 January 1838 (see formannskapsdistrikt). The borders have not changed since.
The municipality (originally the parish) is named after the old Gjerstad farm (Old Norse: Geirreksstaðir), since the first church was built there. The first element is the genitive case of the male name Geirrekr (Debatable! There is no known example of Geirr - Geirrek as a first name from the periode when the farm supposedly got its name. A more likely explanation of Gjerstad is from Geirr - spear. There are over 25 Gjerstad-farms i Norway.) and the last element is staðir which means "homestead" or "farm".