It did not name the owner, but said on its website that the 84,000-cbm LPG ships will be delivered in 2017.
The vessels are 226 metres long and 36.6m wide. They will be built at the Geoje yard.
The contract brings its orders this year to 27 ships worth $3.9bn, against a background of huge losses.
Earlier this month, the country's financial regulator said that state-run Korea Development Bank (KDB) will sell its stake in loss-making shipbuilder DSME.
