Aust-Agder (Norwegian: [ˈæ̂ʉstˌɑɡdər] , English: "East Agder") was a county (fylke) in Norway from 1 January 1919 to 31 December 2019, after 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.