Sirdal is a municipality in the county of Vest-Agder, Norway. It is the northernmost valley in Vest-Agder. At the north end of its long narrow lake, Siredalsvatn, lies the administrative center in the village of Tonstad. Bjørnestad is another village lying in Sirdal. It was formed by the 1964 merger of Øvre Sirdal and Tonstad municipalities, as well as Haugomgrenda.