On 20 July 2005, Boruc signed with Scottish club Celtic. The deal was initially a year’s loan, with an option to make it permanent. The move was later made permanent, with Boruc penning a three-and-a-half-year contract on 17 October until 2009. On 5 January 2008, it was reported that Boruc had signed a new contract with Celtic set to keep him at the club until 2011.
Boruc made his debut for Celtic on 2 August 2005 when he replaced David Marshall for the second leg of their UEFA Champions League qualifier tie against Slovakian side Artmedia Bratislava. Celtic won 4–0, but having lost the first leg 5–0 still went out on aggregate. Boruc quickly established himself as first-choice keeper at Celtic, displaying a combination of outstanding shot-stopping, immense self-belief and, at times, outright arrogance. Indeed, gestures allegedly made by Boruc towards Rangers fans at Ibrox Stadium on 12 February 2006 later resulted in the player being cautioned by the police. In his first season, he made 40 appearances and kept 17 clean sheets. Boruc also added to his winner’s medals collection as Celtic won the Scottish Premier League and the Scottish League Cup, the latter after beating Dunfermline Athletic 3–0.
Boruc continued his fine form for Celtic the following season. On 21 November 2006, in a UEFA Champions League match against Manchester United at Celtic Park, Boruc helped Celtic through to the knockout stage of the competition by saving Louis Saha’s 89th-minute penalty. Indeed, Boruc succeeded in keeping a clean sheet in each of Celtic’s three home Champions League group stages games (Copenhagen 1–0, Benfica 3–0 and Manchester United 1–0). He was named player of the month for December 2006 in the Scottish Premier League, a very uncommon event for a goalkeeper. His form was also recognised with a nomination for the Scottish PFA Players’ Player of the Year award for 2007. The season culminated in more silverware for Boruc as Celtic retained their Scottish Premier League title and defeated Dunfermline 1–0 on 26 May 2007 to win the Scottish Cup.
The start of the 2007–08 season saw Celtic drawn against Russian side Spartak Moscow in the third qualifying round for the Champions League. The tie ended in a penalty shootout, with Boruc saving twice to ensure Celtic’s passage to the group stages. The season also saw Celtic complete a hat-trick of consecutive Scottish Premier League titles.
Boruc was fined an estimated £50,000 by Celtic in August 2008 for breaching the club’s drinking policy after a pre-season friendly in the Netherlands against Feyenoord.
Boruc endured a decline of form in the first half of season 2008–09 as he committed several goalkeeping errors, in particular against Rangers in a 2–4 defeat at Parkhead on 31 August 2008 where he was badly at fault for Rangers fourth goal, and against Hibernian at Easter Road on 7 December 2008, where John Rankin scored against him from 45 yards out, a goal Rankin later referred to as a “squiggler.” Despite this run of relatively poor performances, Boruc was one of 55 players (including five goalkeepers) shortlisted on 20 October 2008 for the FIFPro World XI Player Awards. Discipline, however, also became an issue again, with Boruc fined £500 by the Scottish Football Association (SFA) for gestures he made towards Rangers fans during the Old Firm game in August.
On 28 January 2009, Boruc converted a penalty for Celtic during the penalty shootout in the semi-final match of the League Cup against Dundee United, helping Celtic to an 11–10 win on penalties. Boruc picked up his last silverware at Celtic in the same tournament a couple of months later when Celtic defeated Rangers 2–0 after extra-time in the Scottish League Cup Final.
On 12 December 2009, Boruc made his 200th appearance for Celtic against Motherwell, which Celtic won 3–2. In April 2010, he won the Scottish Premier League save of the season award for a reflex save against Hibernian at Easter Road in August 2009.
On 4 May 2010, Boruc made the last appearance of his Celtic career in a 2–1 victory against Old Firm rivals Rangers, although he had to be substituted in the second half as he was unable to continue playing due to an injury he sustained to his hand in the pre-match warm up. This transpired to be a broken finger and ruled him out of the final league fixture of the season.