As the Messi family had relatives in Catalonia, they sought to arrange a trial with Barcelona in September 2000. First team director Charly Rexach immediately wanted to sign him, but the board of directors hesitated; at the time it was highly unusual for European clubs to sign foreign players of such a young age. On 14 December, an ultimatum was issued for Barcelona to prove their commitment, and Rexach, with no other paper at hand, offered a contract on a paper napkin.^[12][14] In February 2001, the family relocated to Barcelona, where they moved into an apartment nearby the club's stadium, Camp Nou. During his first year in Spain, Messi rarely played with theInfantiles due to a transfer conflict with Newell's; as a foreigner, he could only be fielded in friendlies and the Catalan league. Without football, he struggled to integrate into the team; already reserved by nature, he was so quiet that his teammates initially believed he was mute. At home, he suffered from homesickness after his mother moved back to Rosario with his brothers and little sister, María Sol, while he stayed in Barcelona with his father.^[9][14][15]Lionel Andrés "Leo" Messi (Spanish pronunciation: [ljoˈnel anˈdɾes ˈmesi] ( listen); born 24 June 1987) is an Argentine professional footballer who plays as a forward for Spanish club Barcelona and the Argentina national team. Often considered the best player in the world and rated by many in the sport as the greatest of all time, Messi is the only football player in history to win four FIFA Ballons d'Or, all of which he won consecutively. As the all-time top goalscorer in La Liga, he is the first player to win three European Golden Shoes as the continent's league top scorer and the only player to be top scorer in five Champions League seasons. With Barcelona, he has won La Liga seven times and the UEFA Champions League four times, as well as three Copas del Rey.

Born and raised in central Argentina, Messi was diagnosed with a growth hormone deficiency as a child. At age 13, he relocated to Spain to join Barcelona, who agreed to pay for his medical treatment. After a fast progression through Barcelona's youth academy, Messi made his competitive debut aged 17 in October 2004. Despite being injury-prone during his early career, he established himself as an integral player for the club within the next three years, finishing 2007 as a finalist for both the Ballon d'Or and FIFA World Player of the Year award, a feat he repeated the following year. His first uninterrupted campaign came in the 2008–09 season, during which he helped Barcelona achieve the first treble in Spanish football. At 22 years old, Messi won the Ballon d'Or and FIFA World Player of the Year award by record voting margins.Throughout his career, Messi has been involved in charitable efforts aimed at vulnerable children, a commitment that stems in part from the medical difficulties he faced in his own childhood. Since 2004, he has contributed his time and finances to the United Nations Children's Fund (UNICEF), an organisation with which Barcelona also have a strong association.^[328][329] Messi has served as a UNICEF goodwill ambassador since his appointment in March 2010, completing his first field mission for the organisation four months later as he travelled to Haiti to bring public awareness to the plight of the country's children in the wake of the recent earthquake. He has since participated in UNICEF campaigns targeting HIV prevention, education, and the social inclusion of disabled children.^[330]To celebrate his son's first birthday, in November 2013, Messi and Thiago were part of a publicity campaign to raise awareness of mortality rates among disadvantaged children.^[331]

In addition to his work with UNICEF, Messi founded his own charitable organisation, the Leo Messi Foundation, which supports access to health care, education, and sport for children.^[332] It was established in 2007 following a visit Messi paid to a hospital for terminally ill children in Boston, an experience that resonated with him to the point that he decided to reinvest part of his earnings into society.^[326] Through his foundation, Messi has awarded research grants, financed medical training, and invested in the development of medical centres and projects in Argentina, Spain, and elsewhere in the world.^[326][333] In addition to his own fundraising activities, such as his global "Messi and Friends" football matches, his foundation receives financial support from various companies to which he has assigned his name in endorsement agreements, with Adidas as their main sponsor.^[334][335]

Messi has also invested in youth football in Argentina: he financially supports Sarmiento, a football club based in the Rosario neighbourhood where he was born, committing in 2013 to the refurbishment of their facilities and the installation of all-weather pitches, and funds the management of several youth players at Newell's Old Boys and rival club Rosario Central, as well as at River Plate and Boca Juniors in Buenos Aires.^[326] At Newell's Old Boys, his boyhood club, he funded the 2012 construction of a new gymnasium and a dormitory inside the club's stadium for their youth academy. His former youth coach at Newell's, Ernesto Vecchio, is employed by the Leo Messi Foundation as a talent scout for young players.^[9]