is a municipality in Agder county, Norway. It is located in the traditional district of Lister. The administrative centre of the municipality is the town of Alleen. Some of the main villages in Lyngdal include Austad, Byremo, Fleseland, Hæåk, Konsmo, Korshamn, Kvås, Skomrak, Svenevik, and Vivlemo.
The municipal economy centers around wood processing, agriculture, and commerce. Tourism is also central to the community, with the beaches along the Lyngdalsfjorden and Rosfjorden being popular resorts during the summer.
The municipality is the 177th largest by area out of the 356 municipalities in Norway. Lyngdal is the 108th most populous municipality in Norway with a population of 10,480. The municipality's population density is and its population has increased by 9.3% over the previous 10-year period.
The parish of Aa was established as the municipality of Lyngdal on 1 January 1838 (see formannskapsdistrikt law). On 1 January 1909, the municipality was split into three: Austad in the south (population: 1,263), Kvås in the north (population: 736) and Lyngdal in the central area (population: 2,698). During the 1960s, there were many municipal mergers across Norway due to the work of the Schei Committee. On 1 January 1964, the municipalities of Austad (population: 608), Kvås (population: 493), Lyngdal (population: 2,916) as well as the Gitlevåg area (population: 103) in Spangereid municipality, west of Lenesfjorden, were all merged to form one large municipality of Lyngdal. On 1 January 1971, the Ytre Skarstein and Indre Skarstein areas of Lyngdal (population: 21) was transferred to the neighboring municipality of Farsund. On 1 January 2001, the municipality of Lyngdal declared that the municipal centre of Alleen would be a town called Lyngdal.
On 1 January 2020, the neighboring municipality of Audnedal was merged into Lyngdal municipality.
The municipality is named after the Lyngdalen valley (Lygnudalr) since it is the valley in which the municipality is located. The first element is the genitive case of the river name Lygna.