The Borough of Islwyn was one of five local government districts of Gwent from 1974 to 1996.
The district was formed under the Local Government Act 1972 from part of the administrative county of Monmouthshire, namely the urban districts of Abercarn, Mynyddislwyn and Risca, and the Bedwellty urban district less the wards of Aberbargoed, Cwmsyfiog, New Tredegar and Phillipstown.
The district's name (meaning "below the grove") was derived from the ancient parish of Mynyddislwyn which covered its area. This was shown in the borough's coat of arms which represented a mountain below a grove of oak trees. Islwyn was also the nom de plume of local poet William Thomas (1832-1878).
The borough was abolished in 1996, when its area became part of Caerphilly County Borough.
Islwyn continues as a Westminster constituency and a Welsh Assembly constituency.