Frederikssund is a municipality (Danish, kommune) on the shores of Roskilde Fjord in Frederiksborg County in the northern part of the island of Zealand (Sjælland) in eastern Denmark. The municipality covers an area of 41 km², and has a total population of 18,640 (2005). Its mayor is Knud B. Christoffersen, a member of the Social Democrats (Socialdemokraterne) political party.

The site of its municipal council is the town of Frederikssund.

Neighboring municipalities are Ølstykke, Slangerup, and Skævinge to the east, Frederiksværk to the north, Jægerspris and Skibby to the west (separated by the waters of Roskilde Fjord), and Bramsnæs to the south.

A bridge crossing Roskilde Fjord at the town of Frederikssund connects Frederikssund municipality to Jægerspris municipality.

By January 1, 2007 Frederikssund municipality will cease to exist as the result of ''Kommunalreformen'' ("The Municipality Reform" of 2007). It will be merged with existing Jægerspris, Slangerup, and Skibby municipalities to form the new Frederikssund municipality. This will create a municipality with an area of 260 km² and a total population of ca. 44,140. The new municipality will belong to the new Region Hovedstaden ("Capital Region").

Cultural institutions are the Willumsen Museum and the annual viking plays. Favorite summertime activities include sailing, bicycling, eating ice cream, golfing, and sitting outdoors on the main pedestrian street (gågade).