Celtic are always searching for value in the transfer market. Their entire business model revolves around signing a player on the cheap, providing them with a platform on which to perform, before selling the player on for an enormous profit.
Jota, Nicolas Kühn, Moussa Dembélé, Odsonne Édouard, Matt O'Riley, Kristoffer Ajer and Virgil van Dijk are just some of the recent high-profile examples of this model working.
The Scottish champions are not adverse to spending a bit of money in the transfer market, but they also love bringing in a player on a Bosman. Alex Oxlade-Chamberlain's arrival in February 2026 makes him the latest player to join on a free. So, here are the club's top five free transfers since the Bosman ruling in the mid-90s.
5) Joe Ledley
Back in the summer of 2010, now a free agent, midfielder Joe Ledley reportedly turned down Roma, Rangers, West Brom and Stoke to sign for Celtic from Cardiff. Well, the Welsh international would go on to make 157 appearances in Hoops, winning three Premiership titles before departing for Crystal Palace in 2014.
His most memorable moment in Glasgow came during his second season, heading home the night's only goal as Celtic beat Rangers in an Old Firm derby just after Christmas. Given that the Gers liquidated soon after, this towering header lives long in the memory.
4) Charlie Mulgrew
Also signing as a free agent in July 2010 was Charlie Mulgrew. Having been released by Celtic as a 20-year-old just four years earlier, Mulgrew had impressed at Aberdeen, so he was re-signed by Neil Lennon. He would make 208 appearances for the Hoops in total during six seasons at the club, picking up five Premiership titles.
Mulgrew was part of the team that famously beat Barcelona 2-1 in the Champions League in November 2012, his corner headed in by Victor Wanyama to break the deadlock. It was not just corners that the 6 ft 3 in (191 cms) defender would stand over. He is a rare free-kick scoring centre-back, with six of his 29 goals for the club coming from dead balls, as well as many more long-range screamers.
3) Aaron Mooy
Aaron Mooy was not a Celtic player for a long time, but he sure made an impact. Having joined Shànghǎi Port FC in 2020, he spent the entire Covid-19 period in China, the country hit hardest by the pandemic of course. Thus, during those two years, there was very little football for him to play, so it was certainly a gamble when fellow Aussie Ange Postecoglou signed him in the summer of 2022.
However, Mooy's quality was clear for all to see. In 42 appearances for the Celts, he scored seven goals and registered 11 assists, most notably bagging a brace at Easter Road during a 4-0 win over Hibernian. He missed the Scottish Cup Final against Inverness Caley Thistle and announced his retirement from football after just one season in Glasgow, but what a campaign it was for both Mooy and the team in general.
2) Craig Gordon
We like to think of footballers as superhumans, and in Craig Gordon's case, that is very much applicable. At 43 years old, the goalkeeper is still playing for Hearts, very much in with a chance of being Scotland's number one at the World Cup this summer, having started the final two qualifiers. This is despite the fact that he has suffered not one but two career-threatening injuries.
In 2011, while at Sunderland, Gordon ruptured his anterior cruciate ligament, thereby spending around three years on the sidelines. He then suffered a broken leg at Tannadice on Christmas Eve 2022, returning 13 months later, and still going strong to this day.
After his first serious injury, Celtic were the club that offered Gordon a route back into professional football. Following the sale of Fraser Forster to Southampton for £10 million, Gordon arrived in 2014 and would make 242 appearances for the club, keeping 121 clean sheets, which isn't bad. His celebration when Tom Rogic scored the winner in the pouring rain against Aberdeen in the 2017 Scottish Cup Final will always be an iconic image, a key figure in the historic quadruple treble winning team, and he will always be a Scottish football legend.
1) Mikael Lustig
Having said all of that, Gordon does not take the top spot in this ranking. That honour goes to cult hero Mikael Lustig. Out of contract at Rosenborg, the right-back arrived at Celtic on New Year's Day 2012, and would be a stalwart in the Celts side for the next seven and a half years.
Lustig was first-choice throughout his time at Parkhead, accumulating 276 appearances, picking up 16 major trophies, never failing to land the Premiership title. Many of his 21 goals are iconic, as was an assist at McDiarmid Park in February 2017, his rabona finding Moussa Dembélé to complete an unbroken 25-pass move against St Johnstone, a goal so good it was shortlisted for the FIFA Puskás Award. Any subsequent right-back, including Josip Juranović and Alistair Johnston, has been tasked with replicating Lustig, with the Swede's legacy living on.