Guldborgsund is a municipality (Danish, kommune) in Region Sjælland in Denmark, spanning the Guldborgsund strait. It covers an area of 907 km² and a total population of 63,496 (2008). Its administrative seat is in the town of Nykøbing Falster. Its mayor as of 2010 is John Brædder from the local party New Guldborgsund.
On January 1, 2007 Guldborgsund municipality was created as the result of Kommunalreformen ("The Municipal Reform" of 2007), consisting of the former municipalities of Nykøbing Falster, Nysted, Nørre Alslev, Sakskøbing, Stubbekøbing, and Sydfalster.
The largest company headquartered in the municipality is Codan Medical in Nysted, although its activities are actually concentrated in Rødby Havn in neighbouring Lolland Municipality.
Coordinates: 54°46′11″N 11°52′57″E / 54.769722°N 11.8825°E