As of the end of May, 5G subscriptions stood at 15.84 million, accounting for 22 percent of the total 71.45 million mobile subscriptions in the country, according to the data from the Ministry of Science and ICT.
South Korea has a population of around 51 million.
Top mobile carrier SK Telecom had the most 5G subscribers at 7.39 million, followed by KT Corp at 4.8 million and LG Uplus Corp. at 3.6 million.
User growth on 5G networks has been relatively sluggish in recent months amid a lack of new flagship devices, reports Yonhap news agency.
Samsung Electronics, the top smartphone maker, is expected to launch its new foldable smartphones in August.
The latest figure also comes as telecom operators face a lawsuit from 5G users who complain over disappointing service quality.
Around 500 5G users have taken collective legal action this week against the three carriers, seeking compensation for poor network connectivity and disappointing speeds.
Telecom operators have been racing to improve services, aiming for nationwide coverage by the end of next year.
They are also testing new services with 5G on the 28 gigahertz band, which theoretically allows for speeds that are 20 times faster than 4G LTE. Download speeds on 5G last year were around four times faster than 4G LTE.
Telecom operators hope to power new tech services, such as live virtual reality feeds of sports matches and delivery robots, with high-band 5G.
South Korea's telecom operators first launched 5G networks in April 2019.