Frederikssund is a Danish town, seat of the Frederikssund Municipality, in the Region Hovedstaden with a population of 17,135. It received the status of market town in 1810. The town is famous for its annual Viking Games as well as for the J.F. Willumsen museum. Since 1935, it has been connected to Hornsherred via the Kronprins Frederik Bridge.

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.