Caerdydd Ffynnon Taf (Welsh for 'Cardiff [and] Taff's Well'; listen^ⓘ) is a proposed six-member constituency of the Senedd (Welsh Parliament; Welsh: Senedd Cymru) to be used for the 2026 Senedd election.

It was proposed following the 2026 review of Senedd constituencies, and will be a pairing of the two UK Parliament constituencies of Cardiff North and Cardiff East. It is to have a Welsh-only name.

A Senedd constituency comprising the boundaries of the UK Parliament constituencies of Cardiff North and Cardiff East, has been proposed by the Democracy and Boundary Commission Cymru for the 2026 election to the Senedd (Welsh Parliament; Senedd Cymru). It was initially proposed using the English name Cardiff East and North in September 2024.^[1] The pairing was then switched from Cardiff East to Cardiff West and renamed to Cardiff North-west, and its Welsh name being Gogledd-orllewin Caerdydd in December proposals, despite most other constituencies using Welsh-only names.^[2] It was later given the Welsh-only name Caerdydd Ffynnon Taf and its boundaries were reverted back to using Cardiff East rather than Cardiff West in the commission's final recommendations in March 2025.^[3]

It encompasses parts of the principal areas (counties/county boroughs) of Cardiff and Rhondda Cynon Taf 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.^[1]