The 1915 Merthyr Tydfil by-election was a parliamentary by-election held on 25 November 1915 for the British House of Commons constituency of Merthyr Tydfil in Glamorganshire, Wales.
The seat had become vacant when the constituency's Labour Member of Parliament (MP), Keir Hardie, died on 26 September 1915, aged 59. He had held the seat since the 1900 general election, when he was elected as one of the first two Labour MPs.