Gundsø was a municipality (Danish, kommune) in Roskilde county on the island of Zealand (Sjælland) in East Denmark.
The area of Gundsø municipality was 64 km2, with a total population of 15,749 (2005).
Towns in the municipality included Herringløse, Gundsølille, Gundsømagle, Jyllinge (the municipality's largest town), Østrup Holme, and Ågerup.
The municipality was created in 1970 due to a kommunalreform ("Municipality Reform") that combined a number of existing parishes:
This created a municipality with an area of 212 km2 and a total population of 79,441 (2005).