Freudenstein, Gothic castle ruin on a hilltop near Jáchymov, Czech Republic.

Freudenstein is a castle ruin on Zámecký vršek hill about one kilometre west of Jáchymov, at 810 metres. Two corner towers survive: the smaller southwest tower, 8 metres in diameter with 1.7-metre-thick walls, and the northeast residential tower, rebuilt in 1973 after a lightning strike. Only a short stretch of the perimeter wall remains.

Stephan Schlick had the castle built between 1517 and 1520, with design by Johann Münnich, to protect and administer the silver mines of Sankt Joachimsthal. In 1525 about 3,000 miners stormed and plundered it during the German Peasants' War, and after 1545 it served as the seat of royal mining captains. Swedish artillery heavily damaged it during the Thirty Years' War, and in 1648 the departing Swedes blew up the remains.

The name Freudenstein is German, not derived from the Schlick family; the castle is also called Schlickburg after the Counts of Schlick. The northeast tower became the town watchtower in 1861, where a watchman rang the miners' bell to mark the start and end of shifts.

The site is located at elevated position and requires a climb, but the paths are generally walkable. Visitors should wear sturdy shoes and be aware that some areas may have limited access due to the condition of the ruins.

The first Joachimsthaler Guldengroschen were probably struck in 1519 in the castle cellars, before a dedicated mint was set up after the privilege of 25 January 1520. Johannes Mathesius recorded this early minting, which later gave the thaler its name.