Often referred to as an Argentine-Spanish player, this crossover was brought into sharp focus when in 2004, Lionel was offered the chance to play for the Spanish national U-20 team. He declined the offer given his Argentinian heritage, and was given the opportunity to play for Argentina's U-20 team in a friendly match againstParaguay in June 2004.^[180] Messi scored once against Paraguay and twice in friendly against Uruguay in July 2004.^[181] In 2005 he was part of a team that finished third in 2005 South American Youth Championship in Colombia. He won the 2005 FIFA World Youth Championship in the Netherlands with the team and won theGolden Ball and the Golden Shoe,^[182] scoring in the last four of Argentina's matches and netting a total of six for the tournament.

He made his full international debut on 17 August 2005 against Hungary at the age of 18. He was substituted on during the 63rd minute, but was sent off on the 65th minute because the referee, Markus Merk, claimed that Messi had allegedly elbowed defender Vilmos Vanczák, who was tugging Messi's shirt. The decision was contentious and Diego Maradona even claimed the decision was pre-meditated. Messi was later reportedly found weeping in the changing rooms after the decision.^[183][184] Messi returned to the team on 3 September in Argentina's 1–0 World Cup qualifier away defeat to Paraguay. Ahead of the match he had said "This is a re-debut. The first one was a bit short."^[185] He then started his first game for Argentina in a World Cup qualifier against Peru, in which he was able to win a crucial penalty for Argentina to win the match; after the match national coach José Pékerman described Messi as "a jewel".^[186] He scored his first goal for Argentina in a friendly match against Croatia on 1 March 2006.