Hesselø is a small island of Denmark, belonging to Halsnæs Municipality, in Region Hovedstaden. The island is situated in the Kattegat, about 25 km north of Sjælland, and has an area of 0.71 km².
The island got its name from the common seal (in Danish, sæl), which formerly were very common on the island.
Since the lighthouse was automated in 1955, the population of the island has descended from 16 to 2 (as of 2000).