It was proposed following the 2026 review of Senedd constituencies, and will be a pairing of the two UK Parliament constituencies of Cardiff West and Cardiff South and Penarth. It is to have a Welsh-only name.
A Senedd constituency comprising the boundaries of the UK Parliament constituencies of Cardiff West and Cardiff South and Penarth, has been proposed by the Democracy and Boundary Commission Cymru for the 2026 election to the Senedd (Welsh Parliament; ). It was initially proposed using the English name Cardiff West, South and Penarth in September 2024. The pairing was then switched from Cardiff West to Cardiff East and renamed to Cardiff South-east Penarth, and its Welsh name being in December proposals, despite most other constituencies using Welsh-only names. It was later given the Welsh-only name and its boundaries were reverted back to using Cardiff West rather than Cardiff East in the commission's final recommendations in March 2025. When announcing their candidates, Reform UK used "Cardiff South-west Penarth" instead, based on the English names for the pair of UK Parliament constituencies that form it.
It encompasses parts of the principal areas (counties/county boroughs) of Cardiff, Rhondda Cynon Taf and the Vale of Glamorgan in South Wales. The constituency is to be established by 2026, following the passing of the Senedd Cymru (Members and Elections) Act 2024. The act legislates electoral reform of the Senedd to create 16 larger "super constituencies", pairing the 32 UK Parliament constituencies in Wales, and using a new fully proportional voting system, with each constituency electing six Members of the Senedd (MSs) rather than one previously.