Aust-Agder (Norwegian: [ˈæ̂ʉstˌɑɡdər] (listen), English: "East 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.
The county, located along the Skagerrak coast, extended from Gjernestangen at Risør to the Kvåsefjorden in Lillesand. The inner parts of the area included Setesdalsheiene and Austheiene. Most of the population lives near the coast; about 78% of the county's inhabitants live in the five coastal municipalities of Arendal, Grimstad, Lillesand, Tvedestrand, and Risør. The rest of the county is sparsely populated. Tourism is important, as Arendal and the other coastal towns are popular attractions.
The county includes the larger islands of Tromøya, Hisøya, Justøya, and Sandøya. The interior of the county encompasses the traditional district of Setesdal, through which the river Otra flows to the coast.
In 2017, the Parliament of Norway voted to merge Aust-Agder and Vest-Agder counties into one large region, Agder, effective 1 January 2020.The county was part of the Aust-Agder District Court and the Church of Norway Diocese of Agder og Telemark.