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".

The coat-of-arms is from modern times. They were granted on 18 April 1986. The arms show three knives as a symbol for the smithies and knife makers in the municipality. Knife making has long been a local tradition for which the village is well known.

Gjerstad was, until about 1650, named Vissedal, from vidr-ser-dalr: wood-sea-dale, a very proper description of the area. Gjerstad has been inhabited from the Stone Age and Viking Age through modern times. Agriculture has long been important, and until only decades ago the more marginal outfields in Upper Gjerstad were still utilized. Forestry also provided an important historic economic contribution, and logs were driven on rivers and streams in the municipality. To aid log transport by floating, numerous dams were constructed. Eikeland Ironworks were a significant contribution to the economy as well.

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