A by-election was held in the United Kingdom Parliament constituency of Chesham and Amersham on 17 June 2021, following the death of the sitting member, Dame Cheryl Gillan, on 4 April 2021. Gillan had served as MP for the constituency since 1992. The by-election was the third to the 58th Parliament, which was elected in 2019. The Liberal Democrat candidate, Sarah Green, won the by-election with 56.7% of the vote and a swing from the Conservatives of 25.2%. This is the first time a non-Conservative candidate has won this seat since its formation in 1974.