Hvalsø is a town and a former municipality in Region Sjælland on the island of Zealand in east Denmark. The former municipality covered …Hvalsø is a town and a former municipality in Region Sjælland on the island of Zealand in east Denmark. The former municipality covered an area of 72 km², and had a total population of 7,856. Its last mayor was Virginia Holst, a member of the Venstre political party. She was the first foreign national becoming mayor in a Danish municipality.