Islwyn (Welsh pronunciation: [ˈɪslʊɨn]) was a constituency in Wales represented in the House of Commons of the Parliament of the United Kingdom.

The area, historically known for coal-mining, had voted for Labour Party candidates since it was created effective at the 1983 general election.

The constituency was abolished as part of the 2023 periodic review of Westminster constituencies and under the June 2023 final recommendations of the Boundary Commission for Wales for the 2024 general election.

Its wards were split between Blaenau Gwent and Rhymney, Caerphilly, and Newport West and Islwyn.

2010–2024: The Caerphilly County Borough electoral divisions of Aberbargoed, Abercarn, Argoed, Blackwood, Cefn Fforest, Crosskeys, Crumlin, Maesycwmmer, Newbridge, Pengam, Penmaen, Pontllanfraith, Risca East, Risca West, and Ynysddu.