Horneck Castle, Medieval castle in Gundelsheim, Germany

Horneck Castle is a medieval fortress above the Neckar River in Gundelsheim, featuring thick stone walls and defensive structures overlooking the surrounding landscape. The buildings display typical castle architecture with towers and fortified chambers that fit into the hilly terrain.

Around 1250, Konrad von Horneck gave the castle and his share of Gundelsheim to the Teutonic Order and joined it with his two sons, becoming the first Komtur of the new Kommende Horneck. In 1438 the Deutschmeister moved their permanent residence nominally to the castle, making it the order's third main seat after Marienburg and Riga. In 1525 rebellious peasants, reportedly on the orders of Götz von Berlichingen, captured and burned the residential and economic buildings; the Deutschmeister fled to Heidelberg and the seat was moved to Mergentheim. The castle was rebuilt as a massive Renaissance structure around the surviving bergfried.

Since 1960 the castle has been the Heimathaus Siebenbürgen of the Transylvanian Saxons, run by the Johannes Honterus aid association. It holds the Transylvanian Museum and the Siebenbürgen Institute's library and archives, documenting German-speaking communities from the Carpathian region. The name Horneck is thought to mean "over the Neckar".

The castle stands directly above the Neckar in Gundelsheim, and the town's old centre lies just below it, so the site is reached on foot from the streets around the market. The grounds now serve the Transylvanian Saxons' Heimathaus Siebenbürgen, with the Transylvanian Museum open to visitors; the building also houses a nursing home, so the residential wings are not open to the public.

The name Horneck is thought to mean "over the Neckar", from the castle's position above the river. The bergfried, the only part believed to date from the castle's founding around 1200, survived a planned demolition in the 1720s because the work was too costly.