St Harmon is a village in Radnorshire, Powys, Wales. The population of the Community at the 2011 census was 593. It is located on the Afon Marteg on the B4518 road running between Llanidloes and Rhayader. The parish church is called Saint Garmon, with many people assuming it has been spelt incorrectly. In fact, the village name of St Harmon is a further Anglicisation of the name St Garmon. St Garmon's Church is known for having the diarist the Reverend Francis Kilvert serve as Vicar there between 1876 and 1877. It did have its own railway station- St Harmons railway station.