Caerfyrddin, also known as Carmarthen, is a constituency of the House of Commons in the UK Parliament, to be first contested at the 2024 United Kingdom general election, following the 2023 Periodic Review of Westminster constituencies.

It previously existed between 1542 and 1997 under the English version of the name, Carmarthen. It was named Carmarthen Boroughs from 1832 to 1918; between 1997 and 2024 it was divided between Carmarthen East and Dinefwr constituency and Carmarthen West and South Pembrokeshire.

Because the seat contained mining areas in the valley of the River Gwendraeth (until the 1980s), much countryside and a high proportion of Welsh speakers, it was fertile territory for the Labour Party, the Conservatives and Plaid Cymru alike. Although the Conservatives never won the seat, they came within 1200 votes of doing so in 1983.

Carmarthen is notable as the first constituency to elect a Plaid Cymru MP, Gwynfor Evans, at a 1966 by-election. Evans was later involved in one of the closest General Election results ever in February 1974, when he lost to the Labour candidate by only three votes.^[1][2] The constituency also shot to fame in the following election in October 1974 as the only seat in the country to see its turnout rise on that of February 1974.

Until 1832, it was a borough constituency consisting of the town of Carmarthen. Between 1832 and 1918 it was a district of boroughs constituency, consisting of Carmarthen itself and Llanelli, and was sometimes called "The Carmarthen Boroughs".

In 1918, the borough was abolished, but the name was transferred to one of the divisions of the county of Carmarthenshire. The constituency was made up of the whole of the county of Carmarthenshire except for the urban area around Llanelli. Notable towns were Carmarthen itself, Ammanford and Llandeilo.

In 1997, the Boundary Commission for Wales recommended an extra seat for Dyfed.^[3] This led to the seat being split two to one between Carmarthen East & Dinefwr and Carmarthen West & South Pembrokeshire.^[4]

The constituency is be re-established as Caerfyrddin as part of the 2023 Periodic Review of Westminster constituencies and under the June 2023 final proposals of the Boundary Commission for Wales for the 2024 United Kingdom general election.^[5]