Oscar Knox was diagnosed with neuroblastoma in 2011. The Co Antrim boy's brave fight against the disease was shared by his parents, Stephen and Leona, on Twitter and he soon became an online phenomenon.
His parents announced his death to more than 53,000 followers of the Wee_Oscar Twitter account this morning: "Our beautiful, amazing and much-loved son, Oscar James Knox gained his angel wings yesterday afternoon. Sleep tight little man Xx."
An appeal by his family raised hundreds of thousands of pounds over the last three years. The campaign won many high-profile friends and memorably Oscar wreaked havoc during a visit to the offices of Northern Ireland's First Minister Peter Robinson and Deputy First Minister Martin McGuinness at Stormont.
He was an enthusiastic Celtic supporter but his campaign was also backed strongly by Glasgow Rangers fans - a measure of the boy's ability to unite.
When Celtic played Belfast side Cliftonville in Glasgow last summer in a Champions League qualifier Oscar was the team mascot.
Celtic chief executive Peter Lawwell said: "This is absolutely devastating news and our thoughts and prayers are with Oscar's parents, Stephen and Leona, and his little sister, Izzie, at this desperately sad time.
"We can't even begin to understand the pain of their loss, but I hope that there will be a small measure of consolation in knowing that there was genuine love and affection for Oscar from the Celtic family throughout the world.
"Oscar's courage throughout his illness was truly inspirational, and it was a real pleasure for everyone at the club to meet Oscar when he was our team mascot last July.
"This is a very sad day for the Celtic family but, more importantly, for the Knox family, and our thoughts and prayers are with them today."