Kwon Oh-joon, 69, offered his resignation at an emergency board meeting, but will stay in the post until the company appoints a new chief, a spokesman said.
Kwon took over as CEO of the country's top steelmaker in March 2014. He was re-appointed in March last year, and his term was set to end in March 2020.
POSCO in January reported a 144 percent surge in fourth-quarter operating profit, helped by steel price gains, but fell short of market estimates as higher raw material costs cut into earnings.
The spokesman denied any link between Kwon's resignation offer and a change of government last May.
(Reporting by Jane Chung and Hyunjoo Jin; Additional reporting by Dahee Kim; Editing by Richard Pullin)
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