The 23-year-old Ki, capped 47 times by his country, has been with the Scottish champions since 2010.
"It will be unfortunate to lose a talented player but that's been our strategy at the club now for the last two or three years," Celtic manager Neil Lennon was quoted as saying on the BBC's website on Monday.
"We cultivate players then move them on. We feel it's good business."
Ki was one of South Korea's best performers as they won an Olympic bronze medal at the recent London Games.