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 January 1, 2013, Middelharnis merged with Goedereede, Dirksland, and Oostflakkee into the new municipality of Goeree-Overflakkee.
Middelharnis has a shopping area called D'n Diek. This shopping center settled itself on a dike which used to be the last barrier for the town from the Haringvliet. The town hosts two public and one Christian high schools. Middelharnis has an inland harbor with connection to the Haringvliet. There is also a small beach which can get very crowded in the summertime. In Middelharnis the old main street has the old municipality building.