The 1934 Merthyr by-election was a parliamentary by-election held on 5 June 1934 for the British House of Commons constituency of Merthyr in Wales.
The seat had become vacant when the Labour Member of Parliament (MP) Richard Wallhead had died on 27 April 1934, aged 64. He had won the seat at the 1922 general election as a Labour candidate. At the 1931 general election, he had been returned as an Independent Labour Party (ILP) candidate, but had rejoined the Labour Party in 1933.
The Labour candidate, S. O. Davies, held the seat for his party.