Mynyddislwyn was a civil parish and urban district in Monmouthshire, south east Wales.
The ancient parish of Mynyddislwyn covered a large part of the lower Ebbw and Sirhowy Valleys.
In 1935 the boundaries were adjusted with Bedwas and Machen and Bedwellty urban districts under a County Review Order.
Mynyddislwyn urban district was abolished in 1974, with its area passing to the borough of Islwyn, in the newly created administrative county of Gwent.
St Tudor's church is part of the Benefice of Upper Islwyn, and has services on the first Sunday of every month.