The brilliant Barca star as has coughed up to complete a huge £26.4m appeal raising funds to help combat the illness in Spain.
Messi, 31, has channelled the funds through his Leo Messi Foundation to aid the building of the SJD Pediatric Cancer Center.
The Argentina star turned up to help oversee the laying of the first stone of the project at the Hospital Sant Joan de Deu in the city.
An fundraising appeal started last year looking to reel in the biggest total in public funding in Spain's history to help finance the building work.
And Messi's donation means it reached the £26.4m target, clearing the way for the development of the largest children's oncology centre in Europe.
Once completed, it will be able to care for and treat 400 patients a year, a major increase on the current numbers able to access the facility.
Messi said: "With this initiative, we continue to fight against the cruel disease that is cancer.
"Collaborating makes this easier. I am very happy to be able to make this dream come true."
Manuel del Castillo, the manager of Hospital Sant Joan de Déu, added: "It has been a very emotional event, but this finale, with the announcement of the donation, was a great moment.
"We were happy to have reached £24m, but the final £2.3m donation has been amazing.
"We are very grateful for the contribution made by the Barça Foundation and the Leo Messi Foundation, as well as those of the other donors, but above all we are grateful for the visibility that they have given us."
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