Halsnæs Municipality is a municipality in the Capital Region of Denmark. The municipality covers a total area of 122.15 km2 (2013), according to Municipal Key Figures, and has a total population of 31,466. The municipality also includes the island Hesselø. Up to 20 km2 of the total area is water, as part of the largest lake in Denmark, Arresø, lies within the municipality. Its name comes from Halsnæs, the peninsula that forms the western part of the municipality.

Frederiksværk is a town with a population of 12,815 in Halsnæs Municipality on Zealand in Region Hovedstaden in Denmark.