Guldborgsund Municipality is a kommune in the Region Sjælland of Denmark, created on 1 January 2007 from six (6) former municipalities on the two islands Lolland in the west and Falster in the east bordering the Guldborgsund strait. It covers an area of 903.15 km2 (2013) and has a total population of 60,231. Its neighboring municipalities are Lolland to the west and Vordingborg to the north. Its administrative seat is in the town of Nykøbing Falster. Its mayor as of 1 January 2022 is Simon Hansen, representing the Social Democrats.

Lolland is the fourth largest island of Denmark, with an area of 1,243 km2 (480 sq mi). Located in the Kattegat, Belts and Sund area, it is part of Region Sjælland. As of 1 January 2022, it has 57,618 inhabitants. Lolland is closely connected to the island of Falster to its east. The locality of Sundby forms a cross-island urban area with Nykøbing Falster, the largest conurbation partially on Lolland. The most populated settlement on Lolland proper is Nakskov.