It said the deal with an unspecified European client calls for delivery of the vessels, which are used for deep-water oil exploration, by March 2012.
They will be used off the coast of Brazil, the firm said in a statement.
Samsung said the deal means it has exceeded its yearly target of orders totalling 15 billion dollars.
It said it expects more offshore orders from European and American clients by the end of the year, bringing the annual total to more than 16 billion.
South Korean yards won half the world's shipbuilding orders in the first half of this year, strengthening the country's dominance in the industry despite a sharp fall in overall global demand.