Fonmon Castle (Welsh: Castell Ffwl-y-mwn) is a fortified medieval castle near the village of Fonmon in the Vale of Glamorgan and a Grade I listed building. With its origins rooted in the 12th century it is today seen as a great architectural rarity, as it is one of few buildings that was drastically remodeled in the 18th century, but not Gothicized. The castle is believed to have remained under the ownership of just two families throughout its history; from Norman times the St Johns and from 1656 the descendants of Colonel Philip Jones.
The origins of Fonmon castle are poorly documented and most of its early history has been discovered through studying its architecture. A popular myth exists that the castle was built by Oliver St John of Fonmon, one of the Twelve Knights of Glamorgan who effected the Norman conquest of Glamorgan. It has since been shown that this was a legendary tale given credence by a “historic” recount in the 1560s by Sir Edward Stradling.