The infections among members of the Shincheonji Church of Jesus, the Temple of the Tabernacle of the Testimony (Shincheonji) are linked to one woman.
The sect in the city of Daegu accounts for 30 of 53 new cases, with officials warning of an unprecedented crisis.
South Korea also confirmed its first death from the disease Covid-19.
The coronavirus, which originated in China's Hubei province, causes pneumonia-like symptoms.
China has now seen 2,118 deaths and recorded nearly 75,000 infections.
On Thursday, Japan said two of its passengers who contracted the coronavirus on board quarantined cruise ship Diamond Princess had died. The man and the woman were in their 80s.
What about the outbreak among sect members?
South Korean health officials believe that the infections are linked to a 61-year-old woman who tested positive for the virus earlier this week.
The Korean Centre for Disease Control says the woman - who has not been identified - had contact with 166 people, who were asked to self-quarantine.
At least 30 of the new Covid-19 infections were on Thursday recorded among sect members in Daegu, a city of about 2.5 million people in the south-east.
The Shincheonji, which has been accused of being a cult, said it had now shut down its Daegu branch and that services in other regions would be held online or individually at home.
The mayor of the city has described the event as an "unprecedented crisis" and urged people in the city to stay indoors. Commanders on a nearby US military base also imposed access restrictions, news agency AFP reported.