Ynysddu is a small village and community in the Sirhowy valley of south-east Wales. It is part of the district of Caerphilly within the historic boundaries of Monmouthshire. It lies between to Cwmfelinfach and Wyllie, 4.3 miles north of the town of Risca and 4 miles south of the market town of Blackwood. It is about 10 minutes by road from the M4 motorway and 20 minutes from the Second Severn Crossing. The population in 2008 was 2,905, increasing to 3,948 at the 2011 Census.