2021 South Korean by-elections

The 2021 South Korean by-elections were held in South Korea on 7 April 2021. The National Election Commission announced on 2 March 2021, that the by-elections would be held for 21 public offices or electoral districts, including 2 Metropolitan mayors, 2 Municipal mayors, 8 Metropolitan Council constituencies, and 9 Municipal Council constituencies. Candidate registration ran from 18 to 19 March, and the list of candidates was confirmed on 26 March.

Voters in the by-elections were able to access early voting from 06:00–18:00 KST on 2 and 3 April. One early voting place was established for each district or county of the region where the re-election was conducted, and voters could vote at any pre-voting place in the district where the re-election was conducted, regardless of their address. But on election day, voters had to vote at their designated polling place, and the voting time was from 6 a.m. to 8 pm.

The election concluded with the mayors of Seoul and Busan, which are the largest city and the second-largest city in South Korea, respectively, being selected via by-election. The two major political parties, the Democratic Party and the People Power Party, were concentrating on these by-elections, as they believed that the outcome would have a great impact on the 2022 presidential election. The Democratic Party's candidate Park Young-sun, endorsed by Kim Jinai (Open Democratic Party) and Cho Jung-hun (Transition Korea), became the unity candidate of the ruling coalition. For the opposition, the People Power Party's Oh Se-hoon became the unity candidate, endorsed by Ahn Cheol-soo (People Party) and Keum Tae-seop (Independent).