Old St Mellons is a village and community on the eastern edge of Cardiff, Wales. Lying to the east of the Rhymney River, it forms part of the historic county of Monmouthshire. It is separated from the modern St Mellons suburb by the main road to Newport – the Newport Road – and which was formerly the A48 road. The population of the community in 2011 was 2,367 and one of the smallest in Cardiff.