Chateau d'Onzain, Medieval castle in Veuzain-sur-Loire, France.

Chateau d'Onzain is a castle with stone walls, towers, and moat structures located in the Loire Valley near Veuzain-sur-Loire. The estate is surrounded by gardens and displays construction features from different periods spanning medieval times to the 1800s.

The first stone was reportedly laid in 1183, and in 1276 Guy de Bury is named "sire d'Onzain" in a charter of the Marmoutier abbey. The English held the fortress from October 1358 to 1380, and in 1381 Hélion de Neillac rebuilt it, keeping a keep whose base is still visible at the north-west of the moat. In 1470 Louis XI had Cardinal de la Balue imprisoned here in an iron cage.

The name Onzain appears in a 1276 charter of the Marmoutier abbey, where the lord Guy is called "sire d'Onzain". The seigneurial chapel vault in the parish church carries the arms of Jeanne de Chambes and Jean de Polignac, who rebuilt the castle and built the church choir and two chapels.

The château is a private residence and cannot be visited; it is seen from the public bank of the Loire, where the pale stone retaining wall rises straight from the water below the three-storey facade. The moat still surrounds the building, and the base of the medieval keep remains visible at its north-western corner.

During the French Wars of Religion, Catherine de Medici used the castle to imprison the prince of Conde in 1562. This episode reveals how important the fortress was to the political struggles of the French court.