King John's Castle, Medieval tower house in Kilmallock, Ireland.

King John's Castle is a four-storey tower house in Kilmallock, a National Monument standing 137 m west of the Collegiate Church of St Peter and St Paul. It is vaulted above the ground floor, and its front and back walls were removed so that a large Gothic arch pierces the building. Some original mullioned windows survive, though most were replaced with 18th-century windows.

The fortress was built in the 15th century and later served as an arsenal and military hospital. These uses show how the structure adapted to shifting needs during conflict and warfare.

The name King John's Castle is probably meant to invite comparison with King John's Castle in Limerick, not to claim royal foundation. The tower was renovated in the 18th or 19th century and served as a blacksmith's workshop, a later use that kept the medieval structure in working life.

The tower stands in Kilmallock, about 137 m west of the Collegiate Church of St Peter and St Paul and 152 m southwest of Kilmallock Abbey, so the two can be visited on one walk. It is a National Monument; the sources record no opening times or interior access, so plan on viewing the exterior.

The large Gothic arch piercing the tower is not an original opening: the front and back walls were removed, leaving the four-storey tower house open through its middle. The structure is vaulted above the ground floor, and a few original mullioned windows survive alongside 18th-century replacements.