2023 Selby and Ainsty by-election

A by-election was held in the United Kingdom parliamentary constituency of Selby and Ainsty in North Yorkshire on 20 July 2023, following the announcement on 10 June that the incumbent Conservative Party member of Parliament (MP), Nigel Adams, was standing down from Parliament with immediate effect. Adams had previously announced his intention to stand down as an MP at the next general election, with the party having selected a candidate to succeed him the previous day. Adams formally confirmed his resignation on 12 June.

Keir Mather of the Labour Party won the seat, setting a record for the largest majority ever overturned by the party in a by-election at the time, even as the Labour candidate in the much more marginal Uxbridge and South Ruislip failed to win a by-election held there on the same day. At the time, this was the second-largest swing to Labour in a by-election since 1945 (behind only Dudley West in 1994), but it was then exceeded in the Tamworth by-election held that October and now stands as the third-largest; the Mid Bedfordshire by-election held the same day would also see Labour overturn a larger majority than in Selby.