The 1966 Carmarthen by-election, was held in Carmarthen, Wales, on 14 July 1966. The contest was significant in that it resulted in the election of Gwynfor Evans, the first ever Plaid Cymru Member of Parliament. Plaid Cymru's victory in the Carmarthen constituency, a seminal moment for Welsh nationalism, was part of a wider process toward Welsh devolution which eventually led to the establishment of the Welsh Assembly in 1999.
The election was caused by the death of Labour Party Member of Parliament Megan Lloyd George.
Gwynfor Evans' surprise win is credited with laying the foundations for Winnie Ewing's victory for the Scottish National Party at the 1967 Hamilton by-election, an event of equal significance for Scottish nationalism.