The Cynon Valley by-election, 1984 was a parliamentary by-election held on 3 May 1984 for the British House of Commons constituency of Cynon Valley.
The seat had become vacant on 10 February 1984. The constituency's Labour Member of Parliament (MP), Ioan Lyonel Evans (July 1927 – 10 February 1984), had died at the age of 56.
Evans was a Labour Co-operative politician. He was educated at Llanelli Grammar School and Swansea University College. He served on the West Bromwich education committee and acted as Labour Party agent for the general elections in 1955 and 1959 in Birmingham Small Heath. He was secretary of Birmingham and District Co-operative Party.
Evans was elected as Labour Co-operative MP for Birmingham Yardley at the 1964 general election, but lost the seat in 1970. He was Comptroller of the Household from 1968 to 1970.
At the February 1974 general election he was returned for the Welsh seat of Aberdare which was abolished in 1983, with Evans taking the new seat of Cynon Valley.