The steel maker has been swapping stakes with firms outside of South Korea such as Nippon Steel (5401.T: Quote, Profile, Research) as it seeks to build alliances that can help it fend off a hostile takeover.
The increased shareholding could help anchor POSCO's management strategy, while supporting its plans to adopt friendly shareholder measures such as share buybacks that would ultimately benefit the firms with which it swaps stakes, analysts said.
Hyundai Heavy Industries could also benefit as it depends on steel makers for the plates used to build vessels.
Both firms are among the KOSPI's top-performing blue chips this year, having rallied to record highs.
Hyundai Heavy shares surged to a record 435,000 won, and were trading up 4.7 percent at 434,500 won as of 0257 GMT, compared with a flat wider market (.KS11: Quote, Profile, Research), bringing its gains this year to some 245 percent.
The shipbuilder said on Wednesday the amount of orders received in August more than doubled from a year earlier.
POSCO surged to a record 686,000 won and its shares were last up 1.6 percent at 681,000 won. Shares have gained around 120 percent so far in 2007 amid expectations for strong earnings in the sector on the back of rising steel prices.