Carlsten (Swedish: Karlstens fästning) is a stone fortress located at Marstrand, on the western coast of Sweden.

The fortress was built on the orders of King Carl X of Sweden following the Treaty of Roskilde, 1658 to protect the newly acquired province of Bohuslän from hostile attacks.

On July 23, 1677, after an attack on the fortifications in Marstrand, Carlsten was conquered by Ulrik Frederik Gyldenløve, the Danish military commander in Norway.

In 1719, the fortress was attacked and besieged for a second time in its history by the Norwegian Vice-Admiral Tordenskjold, falling into enemy hands.

In 1658, under the leadership of Johan Wärnschöld, a makeshift fortification was built in the form of a wooden Wahlen skans, at the top of the mountain above the city's "big weather mill".