Frederikssund is a municipality (Danish, kommune) and a town 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 260 km² and has a population of ca. 44,140.