A seat in the Kanagawa at-large district was left vacant following the resignation of Shigefumi Matsuzawa (independent), who was elected to the House of Councillors in the 2019 election. Matsuzawa resigned from his seat in the House of Councillors to run in the 2021 Yokohama mayoral election, for which he came in sixth. Since the seat was not eligible for a by-election, a merger election was held as a part of this election. This was the first time in 30 years - since the Saitama at-large district held one as part of the 1992 election - that a merger election was held. As a result, the Kanagawa at-large district elected five members this election instead of four, with the fifth-place winner serving for only three years instead of six. Furthermore, in October 2021, Kenji Nakanishi (Liberal Democratic), who was elected to the House of Councillors from the Kanagawa at-large district in the 2016 election, resigned his seat to compete in the 2021 Japanese general election for a seat in the House of Representatives, leaving his seat in the House of Councillors vacant prior to this election as well.
