Middelharnis is a town and former municipality in the western Netherlands, in the province of South Holland, on the island of Goeree-Overflakkee. The town had a population of about 6,800 in 2012. On 1 January 2013, Middelharnis merged with Goedereede, Dirksland, and Oostflakkee into the new municipality of Goeree-Overflakkee.

Goeree-Overflakkee is the southernmost delta island of the province of South Holland, Netherlands. It is separated from Voorne-Putten and Hoeksche Waard by the Haringvliet, from the mainland of North Brabant by the Volkerak, and from Schouwen-Duiveland by Lake Grevelingen.