Aust-Agder (, East Agder) is one of 18 counties (fylker) in Norway, bordering Telemark, Rogaland, and Vest-Agder counties. In 2002, there were 102,945 inhabitants, which is 2.2% of the total population in Norway. Its area is 9212 km2. The administrative center of the county is the town of Arendal.
The county, which is located at the Skagerrak coast, extends from Gjernestangen at Risør to the Kvåsefjorden in Lillesand. The inner parts of the area includes Setesdalsheiene and Austheiene. The majority of the population live 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 called Agder, effective 1 January 2020.
The county is part of the Aust-Agder District Court and the Church of Norway Diocese of Agder og Telemark.
The meaning of the name is "(the) eastern (part of) Agder", since the word aust is the Nynorsk form of "east".
Until 1919, the name of the county was Nedenes amt. The amt was named after the old Nedenes farm (Norse Niðarnes), since this was the seat of the amtmann (County Governor). The first element is the genitive case of the river name Nið (now called Nidelva) and the last element is nes which means "headland". The meaning of the river name is unknown.