Hanwha Ocean sank about 23% on Tuesday after Canada chose Germany's Thyssenkrupp Marine Systems as preferred supplier for its next submarine fleet.
Up to $100 billion over three decades was at stake, making the loss a major setback for the South Korean shipbuilder.
TKMS said its 212CD platform—already shared by Germany and Norway—opens a new phase of defence cooperation among close NATO allies.
Canada's choice also gives Ottawa deeper access to European defence-industrial networks as Donald Trump increases pressure on NATO members over military spending.
South Korean President Lee Jae Myung called the result disappointing but urged persistence, while analysts said the decision reflected NATO ties, Arctic capability and procurement-risk concerns rather than a rebuff to South Korea.