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.
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.
The coat-of-arms is from modern times. They were granted on 12 December 1958. It shows two horizontal golden bars on a red background. They symbolize the lumber trade and the recovery of iron ore that was important for Aust-Agder's growth. There are two bars to represent the two areas of the county: inland and coastal.
The system of municipalities, or kommuner, was established in Norway in 1837, based on previously existing parishes (see formannskapsdistrikt). Norway had been ceded to Sweden by Denmark in 1814, at which it promptly rebelled and won the right of self-rule, though nominally part of Sweden. In 1905, Norway declared total independence. Meanwhile, as the years progressed, the municipalities did not remain the same, but new ones were formed, old ones broken up, and land was transferred. Since the 1990s, Aust-Agder has been divided into 15 municipalities:
Since the census of 1769, Aust-Agder has experienced a steady population growth: from 29,633 to 79,927 in 1900, and to 102,848 in 2001. There was significant emigration to the United States in the 19th century and early 20th century.