Another highlight of his international career was being named the captain of the team for the FIFA World Cup 2014 that was held in Brazil.

While his club career began at Jeju United, Koo’s talents took him to German football with stints at VFL Wolfsburg, FC Augsburg and FSV Mainz between 2011 and 2019.

Koo then moved to Qatar to play for Al Gharafa and Al Khor before rejoining Jeju in 2022.

The 35-year-old battled with injuries throughout the 2024 season and was only able to feature three times for Jeju, leading to the decision to retire.

"My body wouldn't hold up any longer, be it my knees or my ankles," Koo said in a press conference. "In the past, whenever I was injured, I could at least get a sense of when I'd be able to recover and return. 

“In the past couple of years, it was difficult to figure out that timeline. I knew then I could hang up the cleats and not have any regrets.

"It was always my dream to return to Jeju and retire here," Koo continued. "I am really grateful that I was able to realise that dream."

Koo will now embark on a coaching career at his beloved Jeju, working with the youth development program.