Llanidloes is a town and community on the A470 and B4518 roads in Powys, within the historic county boundaries of Montgomeryshire, Wales. The population in 2011 was 2,929, of whom 15% could speak Welsh. It is the third largest settlement in Montgomeryshire, after Newtown and Welshpool.

Bryan Hugh St. John O'Neil was a British archaeologist who became Chief Inspector of Ancient Monuments for England and Wales.