The first round of a by-election was held in Val-d'Oise's 1st constituency on 28 January 2018, with a second round on 4 February because no candidate secured a majority of votes in the first round. The by-election was called due to the invalidation of the election of Isabelle Muller-Quoy, candidate of La République En Marche! (REM), in the June 2017 legislative elections by the Constitutional Council on 16 November 2017. It was the first legislative by-election held during the 15th National Assembly.