Koo Ja-cheol is a South Korean footballer who plays as midfielder for Bundesliga club FC Augsburg and the former captain of South Korean football team.

Blessed with technique and vision, Koo "has an uncanny ability to find the back of the net, often displaying a knack to charge into the opposition penalty box unsighted."

On 30 January 2011, Koo Ja-cheol successfully moved to VfL Wolfsburg during the winter transfer window, signing a three-and-a-half year contract for an undisclosed fee. On 31 January 2012, he moved on loan to the league rivals FC Augsburg. During the loan period, he scored five goals in 15 appearances, and played a significant part in helping Augsburg escape relegation in their maiden season in the Bundesliga. In his second season at Augsburg, Koo was plagued by injuries but still helped the club survive relegation in the limited games he played in. On 18 January 2014, it was announced that Koo penned a four-and-a-half year deal with Mainz 05. He scored his first goal for Mainz in a 2–0 victory over SC Freiburg. On 31 August 2015, Koo rejoined Augsburg. On 6 March 2016, he scored Augsburg's first hat-trick in the Bundesliga, against Bayer 04 Leverkusen.

Koo played a central role in helping South Korea finish third in the 2011 AFC Asian Cup, with five goals and three assists over the tournament, finishing as the tournament's top scorer. During the 2012 London Olympics, Koo played as a central midfielder in the Korean national team. The South Korean team won the bronze medal in football for the first time in its Olympics history and in doing so, became the second Asian team in the Olympics men's football to reach the semi-finals. He was named captain of the national team ahead of the 2014 FIFA World Cup. He was selected for the 2015 Asian Cup and played a decisive role in South Korea's opening match, a 1–0 victory over Oman, and was named man of the match.