Midfielder Park Jong-woo is under investigation by the IOC and football's governing body, FIFA, for displaying the sign with a slogan supporting South Korean sovereignty over disputed islets that are claimed by both his country and Japan. The largely uninhabited islets are called Dokdo by South Koreans and Takeshima by Japanese.
The IOC and FIFA have statutes that prohibit political statements by athletes and players. Olympic officials had asked the South Korean Olympic Committee to take action against Park and that he not be present at the ceremony.
When the men's football medals were presented at Wembley Stadium following Mexico's 2-1 win over Brazil in the gold-medal match, only 17 of the 18 South Korean players were on the field for the ceremony. Park was not among them.
The IOC had begun an investigation into Park's actions, and FIFA said it has opened a separate investigation to discipline the athlete.
South Korea defeated Japan 2-0 in Cardiff, Wales, on Friday, hours after President Lee Myung-bak raised diplomatic tensions by travelling to the islets. The presidential visit prompted Japan to recall its ambassador from Seoul.
Photos of the player holding a sign on the field during the celebration by South Korea were passed along to FIFA to determine if any further disciplinary action will be taken, the governing body said.
South Korea stations a small contingent of police officers on the islets in a show of control, but Japan maintains that the rocks are its territory. Tokyo renewed the claim last month in an annual defence report.
During his visit Friday, Lee reportedly told police officers there that the islets are "worth sacrificing lives for," according to the presidential office.
Japanese Foreign Minister Koichiro Gemba said it was "incomprehensible why (Lee) would make this trip at this time,"
On Aug. 15, South Korea will commemorate the peninsula's independence in 1945 from 35 years of Japanese colonial rule.