The majority of the 397 new infections were in the greater Seoul region.
Virus curbs were tightened in the Seoul region last week and were further expanded to the rest of the country on Sunday.
All beaches have also been closed.
South Korea has reported a total of 17,399 coronavirus infections with 309 deaths.
Seoul
South Korea reported its highest daily number of coronavirus cases since early March on Sunday as authorities warned the country was "on the brink of a nationwide pandemic" and tightened social distancing rules.
