Five-year-old Oscar Knox touched hearts across the country with his courage after being diagnosed with high-risk neuroblastoma.
He became an online sensation after his parents used social networking to raise awareness of his fight against the rare and aggressive childhood cancer.
Oscar, from Belfast, sadly passed away on Thursday.
His family said: "Our beautiful, amazing and much-loved son Oscar James Knox gained his angel wings. Sleep tight."
Celtic players, fans and club staff joined a support campaign to raise money for Oscar to have specialist treatment in the US.
Club manager Neil Lennon paid tribute to the youngster at a pre-match press conference.
He said: "It's a tragedy. This is a young boy who had an influence on a lot of people's lives, particularly here within the Celtic club and the Celtic support. The supporters and the people who knew the family will be absolutely devastated by the news.
"He fought so hard to keep going and this will have touched a lot of people today.
"All of our condolences and thoughts go out to the family."
Oscar led Celtic out in July as team mascot when Cliftonville travelled for their Uefa Champions League qualifier in Glasgow.
Skipper Scott Brown said: "All the lads have met him and we just feel for his family. It's a hard time that they will be going through but our support is with them.
"He was the one that spurred us on as well. He was just great on the day, full of smiles, laughing and joking and running about and that's the way we want to remember him."
Oscar was diagnosed in November 2011 and more than £600,000 was raised to fund specialist treatment for him.
Celtic chief executive Peter Lawwell said the youngster's courage was "inspirational".