LONDON — A South Korean soccer player was barred from receiving his bronze medal at the London Olympics on Saturday for displaying a sign with a political message after a victory over Japan in the third-place game.

The player, Park Jong-woo, held up a sign after South Korea’s 2-0 victory over Japan, claiming South Korean sovereignty over a set of barely inhabitable islands that are also claimed by Japan.

Mr. Park, a midfielder, played all 90 minutes of the game on Friday in Cardiff, Wales, then was photographed carrying a sign that read, “Dokdo is our territory.” The islands, called Dokdo in Korea and Takeshima in Japan, lie in an area of rich fishing grounds and natural gas deposits.