The new cases brought the country's total infections to 10,653, according to Korea Centers for Disease Control and Prevention (KCDC) data cited by Yonhap news agency.
It is the first time since February 20 that the country's new COVID-19 infections came below 20. Over the past five days, the country reported fewer than 30 new patients per day.
The number of South Korea's daily new cases reached a peak of 909 on February 29, but the country has since managed to flatten the curve amid stringent quarantine and social distancing measures.
The country's death toll came to 232, as two more coronavirus deaths were reported on Friday.
The number of patients released from quarantine after making full recoveries stood at 7,937, up 108 from a day earlier, KCDC data showed.
Of the 18 new cases, three cases were reported from the southeastern city of Daegu, once the epicentre of the virus and the surrounding North Gyeongsang Province added two more cases, according to the KCDC.
Seoul and Gyeonggi provinces reported one and four more cases, respectively, it added.
Nine of the fresh infections were from abroad, with four being detected at border checkpoints. The total number of imported cases is now at 992.
In the wake of a surge in the number of imported cases, South Korea has enforced mandatory 14-day self-isolation for all travellers coming from overseas since April 1.
Despite the downward trend, the government has been on high vigilance and called for strict adherence to anti-virus measures and the social distancing campaign.
The country's prime minister, Chung Sye-kyun, asked the public to maintain social distancing as concerns rise over its possible loosening during upcoming holidays, which could span for about a week starting at the end of this month.
From March 22, the government has intensified its drive, forcing all major public facilities, including schools and private gyms, to shut down to stop the spread of the virus.
The government is also expected to announce soon its decision on whether to further extend the guidance, which is supposed to be in effect until Sunday.
( With inputs from ANI )