Hesselø is a small island of Denmark, belonging to Halsnæs Municipality, in Region Hovedstaden. The island is situated in the Kattegat, about 25 nautical miles east of Grenaa on Jutland and 15 nautical miles northwest of Zealand Sjælland, it has an area of 0.71 km².
Hesselø was occupied by the English during the Napoleonic Wars, and afterwards by the infamous pirate captain Sorte Robert (English: Black Robert). When he went out on a raid, according to the legend, he locked his wife in the cellar, so she couldn't escape from him. In the end he killed both her and his daughter before disappearing without a trace.
In fiction, the Norwegian illustrator and author Tor Bomann-Larsen in the comic book '1905. The Duel at Hesselø' wrote that the Swedish and Danish princes duelled here for the unoccupied Norwegian throne.
Another 'duel' occurred in 1983 when the Danish government gave permission for test-drilling for oil north-east of the island against protests from Sweden.
The island is now privately owned and used as a holiday destination. The lighthouse was automated in 1955, and the island is uninhabited since 2005.