The 1912 East Carmarthenshire by-election was a Parliamentary by-election held on 22 August 1912. The constituency returned one Member of Parliament (MP) to the House of Commons of the United Kingdom, elected by the first past the post voting system.

East Carmarthenshire was a county constituency in Carmarthenshire, Wales. 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.