There are now 94 infections linked to a cluster that emerged in Seoul's nightlife district.
The new spike comes after the government began relaxing social distancing rules - and shows just how difficult it is to control infections after returning to normal life.
Authorities have now delayed this week's planned re-opening of schools.
South Korea, through extensive testing and contact tracing, had managed to bring daily cases down to single digits without enforcing a national lockdown.
But contact tracing the new cluster is made more difficult by the fact that some of the clubs draw in customers from the gay community.
Some guests are thought to provide false names or numbers when entering, as homophobia remains widespread in South Korea. Health officials are now offering anonymous tests.