By-elections to the European Parliament in the United Kingdom were held during the period when the United Kingdom used single-member, first-past-the-post constituencies to elect Members of the European Parliament from 1979 to 1999: they were required when a member resigned, died or was disqualified. Six by-elections were held in this period, all retained by the holding party.

The European Parliamentary Elections Act 1999 established a new system of multi-member constituencies elected by a closed party list system based on the regions of England, plus Scotland and Wales. The single transferable vote system was retained in Northern Ireland. Because of the new system, retiring MEPs would be replaced by a different person from the list, so no further by-elections were held.