This is a list of parliamentary by-elections in the United Kingdom held between 1918 and 1931, with the names of the incumbent and victor and their respective parties. Where seats changed political party at the election, the result is highlighted: red for a Labour gain, blue for a Conservative gain, orange for a Liberal gain, and grey for any other gain. A total of 233 by-elections were held during this period.
Where the cause of by-election is given as "resignation" or "seeks re-election", this indicates that the incumbent was appointed on his or her own request to an "office of profit under the Crown", either the Steward of the Chiltern Hundreds or the Steward of the Manor of Northstead. These appointments are made as a constitutional device for leaving the House of Commons, whose Members are not permitted to resign.
The Succession to the Crown Act 1707 required ministers to seek re-election to the House of Commons on their appointment to office. The Re-Election of Ministers Act 1919 ended the necessity to seek re-election within nine months of a general election, and the Re-Election of Ministers Act (1919) Amendment Act 1926 ended the practice in all other cases.