The 1912 Carmarthen Boroughs by-election was a Parliamentary by-election held in the United Kingdom on 29 January 1912 for the Carmarthen Boroughs constituency in Wales. The constituency of Carmarthen Boroughs was centred on the boroughs of Carmarthen and Llanelli. It returned one Member of Parliament (MP) to the House of Commons of the Parliament of the United Kingdom, elected by the first past the post voting system.