Posco shares closed the day down 2.7 percent at 238,000 won ($216) -- the lowest level since 2006 -- extending its slide for the year to 14 percent.
The sell-off was triggered by a report in the Maeil newspaper that Berkshire Hathaway had sold its estimated 4.5-5.0 percent stake in Posco in the second quarter of last year.
South Korean media have reported prosecutors are examining Posco's past mergers and acquisitions, and its CEO said last month that the company was cooperating with an investigation into its business practices.
The steelmaker is the fifth largest globally.