On 25 August 2009, it was revealed that contact between Celtic and FC Seoul had occurred regarding the possibility of Ki’s transfer to the Parkhead club. However, the player’s agent stated that an immediate move would be unlikely given FC Seoul’s success in the league and the Asian Champions League. Three days later, Celtic clinched a £2.1m transfer for Ki. He linked up with the Parkhead side in the January transfer window at the end of the K-League season. The signing was confirmed on 13 December 2009 after Ki passed a medical and secured a work permit. Ki reportedly turned down an offer from English club Portsmouth then of the Premier League. He took the number 18 with “Ki” on his shirt. He made his debut for Celtic in a 1–1 draw against Falkirk at Celtic Park on 16 January 2010, winning the Man of the Match award from the official Celtic website. He played a further four games for Celtic that season, but struggled to make much impact in what was a period of upheaval for the club.
Ki barely played for Celtic in the first month of season 2010–11, but came off the bench to score his first competitive goal for Celtic on 22 August 2010 with a shot from 25 yards in a 4–0 victory against St Mirren in the Scottish Premier League (SPL). By the end of October he had become a first team regular and was voted the SPL Young Player of the Month for October 2010.
Ki scored his second goal of the season in a 2–2 draw with Inverness Thistle at Celtic Park. On 26 December 2010, Ki scored against St Johnstone in a 2–0 victory in the SPL, in what was his last game for Celtic before travelling to Qatar to play for South Korea in the 2011 AFC Asian Cup during January. On coming back to Scotland in February, Ki returned to the Celtic first team for the 2–2 draw against Rangers in a Scottish Cup tie at Ibrox. Celtic played a significant part of the game with only ten men after Fraser Forster was sent off, but the midfield including Ki won praise for the manner in which they dominated the game in the second half. Ki also played in the replay at Celtic Park, an ill-tempered match which Celtic won 1–0. On 21 May 2011, Ki scored the first goal of Celtic’s Scottish Cup Final win against Motherwell with a left-footed strike from around 35 yards. He also won the official Man of the Match award.
Ki started the 2011–12 campaign in Celtic’s opening game against Hibernian in the SPL. Scoring the second goal in a 2–0 victory, a 25-yard left foot strike into the right hand bottom corner and also won the Man of the Match award from the official Celtic website.
Ki then went on to score another goal on 13 August 2011 in the Scottish Premier League in a 5–1 win over Dundee United at Celtic Park, a strike into the top left hand corner from the edge of the box. A couple of days later it was reported that Premier League sides Blackburn Rovers, Tottenham Hotspur and several clubs from the Russian Premier League were interested in signing Ki. Celtic the next day issued a hands off warning to the interested clubs, saying that it would take a bid of significant proportions for Celtic to allow the increasingly important midfielder to leave the club. On 10 September 2011, Ki put Motherwell to the sword yet again, this time in the Scottish Premier League with a superb strike from outside the box with his right foot. Celtic went on to win the game 4–0. On 29 September, he started in Celtic’s 1–1 draw with Italian side Udinese, scoring on a penalty after 3 minutes. On 18 December 2011, he scored the second goal of the game as Celtic beat St Johnstone 2–0 at McDiarmid Park in the Scottish Premier League. Over the course of the 2011–2012 campaign, Ki scored 7 goals and had 6 assists, playing a key role in helping Celtic win the Scottish title.