The number of smartphone subscribers hit 20.1 million late last week in a country of just under 49 million people -- compared to 10 million in March this year and fewer than 500,000 only two years ago.
With free wireless networks and cutting-edge gadgets widely available, users will likely number 30 million in the first half of 2012 and 40 million at the end of next year, said Lee Sang-Hak, director of the Korea Communications Commission's telecommunications policy planning division.
"We'll likely exceed the US in terms of the percentage of smartphone users in the total population early next year," he told AFP.
South Koreans got their first taste of Apple's iPhone only in November 2009, a year or two after the United States and Europe, when regulators dropped restrictions which had largely closed off the local market.
The iconic device became an instant hit, unnerving Korean cellphone giants like Samsung Electronics and LG Electronics which rushed to roll out competing models.
Samsung, the world's second-largest maker of all types of mobile phones after Nokia, unveiled its flagship Galaxy S smartphone series in June 2010.
In the third quarter of this year it even overtook Apple in the global smartphone market, shipping almost 28 million units compared to 17 million by the US technology giant, according to research firm Strategy Analytics.
Lee said Samsung's success has helped fuel the local smartphone craze.
"Having such a big player at home helps a lot, since Samsung can market, sell and distribute more easily and faster here than elsewhere," he said.
The country's widespread broadband network also helped drive growth, he said, as existing networks can offer wi-fi services and let smartphone users access the Internet with no extra data charges.
South Korea is one of the world's most wired societies, with 95 per cent of homes using broadband Internet. It also has the world's top Internet download speeds, according to a study released by Pando Networks.
SK Telecom, the country's leading wireless operator, has invested a record 2.3 trillion won ($2 billion) this year to upgrade networks to handle growing data traffic caused by its own 10 million smartphone users.
Another 6.8 million use KT, and 3.3 million subscribe to LG Uplus.
"We didn't expect that the number would rise at this speed... investment in network infrastructure is one of our top priorities now," said an SK Telecom spokesman.
Other wireless operators have also strengthened networks, and the capital city joined in as well. In June it pledged to spend $44 million to offer free wi-fi in 10,430 locations -- parks, streets and most public places -- by 2015.
Kang Jeong-Soo, a researcher at the Yonsei Communications Institute in Seoul, said Koreans' passion for gadgets means they waste no time trying out new technologies.
"Average South Korean consumers replace mobile phones every nine months, compared to more than two years in countries like Germany or France," he told AFP.
Many South Korean mobile users are far more sensitive to trends than to prices and are hardly discouraged by higher prices of smartphones or costlier phone bills, he added.
"No one here wants to be left out when a new big technology comes out... especially when everyone else is mingling on Twitter or Kakao Talk," he said, referring to a local mobile messenger app which claims 25 million downloads.