Horsfjärden, fjard in Haninge Municipality, Stockholm County, Sweden

Horsfjärden is a fjard off the Baltic Sea near Stockholm, about 20 kilometres long with a surface area of 61.5 square kilometres. It lies between the island of Muskö and the Södertörn mainland, inside the larger Mysingen fjard. Three Swedish naval bases line its shores: Märsgarn, Muskö and Berga.

Horsfjärden gained military importance in the 20th century, leading to the construction of the Berga Naval Base that has defined much of the region's modern character. The base solidified this waterway as a key strategic location in Sweden's coastal defense.

The name comes from the Old Swedish hors, meaning "horse" or "mare". On the mainland shore lies Årsta havsbad, a holiday cottage area begun by architect Sven Wallander, whose first houses HSB built in 1929. The fjard has long served as a base area for Swedish naval forces at Märsgarn, Muskö and Berga.

The best time to visit is during warmer months when ferry services run more frequently along the coast. Be prepared for changing weather and check service schedules in advance, as conditions vary significantly throughout the year.

The name Horsfjärden comes from the Old Swedish hors, meaning horse or mare. The fjard is about 20 kilometres long and covers 61.5 square kilometres. In October 1982 the Swedish navy sealed off the fjard during a submarine hunt, dropping 44 depth charges and 4 mines without result.