Tonstad is a former municipality that was located in the old Vest-Agder county in Norway. The 361-square-kilometre (139 sq mi) municipality existed from 1905 until 1960. It encompassed the southern part of the present-day municipality of Sirdal. The administrative center of the municipality was the village of Tonstad where Tonstad Church is located.

Vest-Agder was one of 18 counties (fylker) in Norway up until 1 January 2020, when it was merged with Aust-Agder to form Agder county. In 2016, there were 182,701 inhabitants, around 3.5% of the total population of Norway. Its area was about 7,277 square kilometres (2,810 sq mi). The county administration was located in its largest city, Kristiansand.