Arendal (ˈɑ̂ːɳɖɑːɫ) 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 (which is also the seat of Agder county). 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.
The municipality is the 273rd largest by area out of the 356 municipalities in Norway. Arendal is the 23rd most populous municipality in Norway with a population of 45,509. The municipality's population density is and its population has increased by 6.3% over the previous 10-year period.
The town of Arendal was established as a municipality on 1 January 1838 (see formannskapsdistrikt law). On 1 January 1875, a small area with 22 inhabitants was transferred from the town to the neighboring municipality of Østre Moland and another small area with 52 residents was transferred to the neighboring municipality of Øyestad.
On 1 January 1902, the rural municipality of Barbu (population: 6,787) was merged into the town of Arendal. In 1944, a small area of Moland with a population of 21 inhabitants was transferred to Arendal as well. On 1 January 1992, the town was vastly expanded. The neighboring rural municipalities of Hisøy (pop: 4,026), Moland (pop: 8,148), Tromøy (pop: 4,711), and Øyestad (pop: 8,679) were all merged with the town of Arendal which had a population of 12,478, bringing the total population of the new municipality of Arendal to 38,042.
The Old Norse form of the name was probably Arnardalr. The first element is the genitive case of ǫrn which means "eagle". The last element is dalr which means "valley" or "dale", thus meaning the "valley of the eagle".
A link for the name also has been theorized to the Vasconic substrate hypothesis, for similarity to placenames like Val d'Aran and Arundel.