"We concluded that opening our rice market under tariff is inevitable and the best option for the future of (the country's) rice industry," said Agriculture Minister Lee Dong Pil.
The switch would come into effect on Jan 1, replacing a quota system that had been in place for 20 years.
A World Trade Organisation (WTO) agreement that allowed the country to import foreign rice under the minimum market access (MMA) expires at the end of this year after it was extended 10 years ago.
Opposition from South Korean farmers had led to the MMA being extended in 2004, but the WTO only agreed to the extension on condition that the annual quota of imported rice be increased.
As a result, total rice imports soared from about 225,000 tonnes in 2005 to 409,000 tonnes in 2014. The current quantity imported accounts for about 9 per cent of the country's rice consumption.
Senior officials they were considering a tariff of up to 500% on imported rice under the new scheme.
MMA imports are currently subject to a 5% tariff, Yonhap News Agency reported.
South Korea has excluded rice from a planned free trade agreement with China to protect domestic producers. Japan has similarly refused to include rice in a Pacific free trade agreement.