Sirdal is a municipality in Agder county, Norway. It is located in the northwestern part of the traditional district of Lister. The administrative centre of the municipality is the village of Tonstad. Other villages in Sirdal include Bjørnestad, Haughom, Kvæven, Lunde, and Tjørhom.

The 1,554-square-kilometre (600 sq mi) municipality is the 52nd largest by area out of the 356 municipalities in Norway. Sirdal is the 290th most populous municipality in Norway with a population of 1,837. The municipality's population density is 1.3 inhabitants per square kilometre (3.4/sq mi) and its population has inmcreased by 0.3% over the previous 10-year period.

The old parish of Sirdal was first established as a municipality in 1849 when it was separated from the large municipality of Bakke. Initially, Sirdal had 1,804 residents. On 1 January 1903, a small area of Sirdal (population: 63) was transferred to the neighboring municipality of Fjotland. On 1 January 1905, the municipality of Sirdal was dissolved and its territories were divided into two new municipalities: Tonstad (population: 828) and Øvre Sirdal (population: 753). These large, sparsely populated municipalities merged along with the Øksendal area of Bakke municipality to recreate the municipality of Sirdal on 1 January 1960. This time, Sirdal started out with a population of 1,426. On 1 January 1987, the Virak and Espetveit areas of northern Flekkefjord (population: 41) were transferred to Sirdal municipality.

The municipality (originally the parish) is named after the Sirdalen valley (Old Norse: Sírudalr) since the valley runs through the municipality. The first element is the genitive case of the river name Síra (now the Sira river). The river name has an unknown meaning, but it could be something like "strong stream". The last element is dalr which means "valley" or "dale".

The coat of arms was granted on 17 January 1986. The official blazon is "Vert, three ptarmigans volant argent" (Norwegian: I grønt tre oppflygande sølv ryper, 2 - 1). This means the arms have a green field (background) and the charge is a willow ptarmigan (Lagopus lagopus). The ptarmigan has a tincture of argent which means it is commonly colored white, but if it is made out of metal, then silver is used. The green color in the field and the ptarmigans were chosen as a symbol for the hills and moorland which make up the largest part of the municipality. The willow ptarmigan is a typical bird for the area. The arms were designed by Stein Davidsen. The municipal flag has the same design as the coat of arms.

The Church of Norway has one parish (sokn) within the municipality of Sirdal. It is part of the Lister og Mandal prosti (deanery) in the Diocese of Agder og Telemark.

Sirdal municipality includes the westernmost valley in Agder county, bordering Rogaland county to the west. To the south, it borders on the municipality of Flekkefjord and to the east is Kvinesdal. Sirdal is the largest municipality by area in Agder. Lakes in the region include Gravatnet, Kulivatnet, Kvifjorden, Øyarvatnet, Rosskreppfjorden, Sirdalsvatnet, and Valevatn. The mountain Urddalsknuten lies at the northern end of the municipality.

In 1990, the mountain road opened between the Sirdalen valley and the Setesdal valley over the Suleskard mountain pass. It connected Sirdal to Brokke in the neighboring municipality of Valle to the east. It shortened the distance from Oslo to Stavanger by 110 kilometres (68 mi) in the summer, and the road passes through barren, glacier-scoured highland. As it passes by the Rosskreppfjorden, it ascends to over 1,000 metres (3,300 ft) above sea level.