Old St Mellons is a village, a community and a civil parish 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 (B4487) – and which was formerly the A48 road.

St Mellons is a district and suburb of eastern Cardiff, the capital city of Wales. Prior to 1996 St Mellons was the name given to the community largely north of Newport Road (B4487) which included the old St Mellons village. After 1996 the old community was divided and renamed as Old St Mellons and Pontprennau, with the newer, much larger area of modern housing and business parks to the south of Newport Road retaining the St Mellons name. Historically in Monmouthshire, St Mellons became part of South Glamorgan and Cardiff in 1974.