Old St Mellons (Welsh: Pentre Llaneirwg) is a village and community (civil parish) on the eastern edge of Cardiff, Wales. It is separated from what is now the modern St Mellons suburb by the B4487 main road to Newport.
Old St Mellons elects a community council, currently (2016) comprising eleven councillors. The village is part of the Cardiff Council ward of Pontprennau & Old St. Mellons, which in turn is part of the UK Parliament/National Assembly for Wales constituences of Cardiff North. The new St Mellons to the south is part of a different ward and parliamentary constituencies.
Known variously as St Melans or Llaneurwg, St Mellons was part of Monmouthshire prior to 1974. It had been governed by the St Mellons Rural District Council from 1894, which included other villages and settlements as far as the edge of Newport.
Old St Mellons is made up largely of 19th-century housing, a village hall and several pubs and shops. The village had a school, converted in 1854 from the 17th-century Poor House, but this was demolished after it closed in the 1980s. There are 15 nationally listed buildings and structures in the village, including St John's College, St Julian's Manor and two public houses. As of 2007 there were four pubs, indicating the importance of St Mellons as a resting point on the route to London. Because St Mellons lay in Monmouthshire until 1974, it also avoided having to abide by the Welsh Sunday Closing Act.