The election watchdog said Wednesday that it will accept registration from candidates running for parliamentary by-elections later this month.
The National Election Commission said the two-day registration will end on Friday.
Four seats are up for grabs in the April 29 by-elections widely seen as a bellwether for next year's general election.
It is the first election face-off since Kim Moo-sung and Moon Jae-in took the helms of the ruling Saenuri Party and the main opposition New Politics Alliance for Democracy, respectively.
Neither Kim nor Moon is running for any of the seats, though they are expected to give their full support to candidates of their parties as the by-elections could affect their own political future, possibly their presidential bids.
Moon, who was defeated by then ruling party candidate Park Geun-hye in the closely contested 2012 election, is a front-runner among potential presidential candidates in opinion polls.
The most interesting race in the by-elections is in district B of Seoul's southern district of Gwanak, where Chung Dong-young, a former opposition presidential candidate, is competing against ruling and opposition party candidates.
Chung, a former three-term lawmaker and presidential candidate of what is now the NPAD, left the party in January to create a new left-leaning party.
NPAD has condemned Chung's move as "opportunistic." Chung dismissed the accusations, claiming he has only followed one political course.
In South Korea, it is not rare for politicians to switch political parties in the run-up to elections to boost their chances of winning.
The other three parliamentary seats are up for grabs in Seongnam, south of Seoul, Incheon, a port city west of Seoul, and the southwestern city of Gwangju. (Yonhap)