Frederikssund Kommune is a municipality on the shores of Roskilde Fjord in the northern part of the island of Zealand (Sjælland) in eastern Denmark. On 1 January 2007, the municipality was enlarged to include the old Jægerspris, Slangerup, and Skibby municipalities. It now covers an area of 250 km2 and has a population of 46,117. As of 1 January 2014 its mayor is John Schmidt Andersen, a member of the agrarian liberal Venstre political party. Frederikssund municipality belongs to Region Hovedstaden.

Zealand at 7,031 km2 is the largest and most populous island in Denmark proper. Zealand had a population of 2,319,705 on 1 January 2020, comprising 40% of the country's population.