Kozí Hrádek, Gothic castle ruin in Sezimovo Ústí, Czech Republic.

Kozí Hrádek is a Gothic castle ruin situated on a hilltop above Sezimovo Ústí, displaying the layout of a medieval defensive structure. The preserved walls and foundations clearly show the outlines of rooms and fortifications dating back to the 14th century.

The castle was built in the 14th century and played an important role in Bohemian history, particularly in the early 15th century. It was destroyed in 1438 during a raid and was never rebuilt.

The name Kozí Hrádek means "Goat Castle"; the German form Ziegenburg carries the same meaning. The Kozský potok stream below the rock spur shares the root. Since 1900 a national pilgrimage honouring Jan Hus has been held at the ruin, and in 1910 Tomáš Garrigue Masaryk spoke at one of these gatherings.

The castle site sits on a hilltop and is accessible on foot, though visitors should be prepared for the climb. Sturdy footwear is recommended since the ground is uneven and the ruins lack paved pathways.

Amateur archaeologist Josef Švehla, a teacher from Staré Tábor, began clearing the ruins in 1899; his work fully exposed the remains and gave them their present appearance. The excavations yielded over 250 finds, now held in the Tábor Museum. Earlier restoration after 1893 was carried out without method and damaged the site.