Aust-Agder was a county (fylke) in Norway until 1 January 2020, when it was merged with Vest-Agder to form Agder county. In 2002, there were 102,945 inhabitants, which was 2.2% of Norway's population. Its area was 9,212 square kilometres (3,557 sq mi). The county's administrative center was the town of Arendal.

Arendal is a municipality in Agder county in southeastern Norway. Arendal belongs to the region of Sørlandet. The administrative centre of the municipality is the city of Arendal. Some of the notable villages in Arendal include Rykene, Eydehavn, Færvik, Strengereid, Kongshavn, Kilsund, Brattekleiv, Torsbudalen, Longum, Saltrød, Staubø, Vrengen, and Kolbjørnsvik. The offices of UNEP/GRID-Arendal are also located in the city of Arendal.