YSGWYDDGWYN (YSGWYDD-WYN), a hamlet, in the parish of GELLYGAER, hundred of CAERPHILLY, county of GLAMORGAN, SOUTH WALES, 5 miles (S. E.) from Merthyr-Tydvil; containing 123 inhabitants. It is situated on the right bank of the river Romney, near its source, and forms the upper portion of the parish, where the ground is bleak and mountainous, the inhabitants thinly scattered, and the right of common generally exercised. There is a meeting-house for a congregation of dissenters.