William John (6 October 1878 – 27 August 1955) was a Welsh Labour Party politician, and a Member of Parliament (MP) for thirty years.
At the 1920 Rhondda West by-election, he was elected as MP for the safe Labour constituency of Rhondda West, and held the seat until he retired from the House of Commons at the 1950 general election.
He was Labour's Deputy Chief Whip from 1942 to 1945, serving in Winston Churchill's war-time coalition government as Comptroller of the Household from 1942 to 1944 and as a Lord of the Treasury from 1944 to 1945.