Neymar signed a five-year contract with club president Sandro Rosell after choosing Barcelona over Real Madrid in an intense bidding war between the fierce Spanish rivals.
Formalities done, the 21-year-old striker jogged onto the Camp Nou pitch for the first time in his new Barcelona kit bearing the No. 11 and the name "Neymar Jr" on the back.
"I am very happy, very moved to be a Barcelona player and fulfill my dream," Neymar told the tens of thousands of fans who turned out to welcome him.
Neymar made it clear that he knows that, at least for now, he will have to play second fiddle to Messi.
"I want to help the team," he said. "I have come to add my part so that Lionel Messi continues to be the best player in the world."
Neymar's flight from Brazil arrived an hour behind schedule at around 1 p.m. local time. Decked out in solid black with sunglasses and a baseball cap, he took a chartered bus straight to Camp Nou where he took his first photograph in front of the blue and burgundy club insignia.
With the traditional thumbs-up welcome shoot done, Neymar stepped forward and waved over the mob of photographers to the hundreds of fans who had gathered wearing Brazil and Barcelona jerseys and chanting his name. He was then rushed through two medical exams and back to the office to sign the deal.
Even before making it official, Neymar's new teammates had given him their vote of confidence.
Earlier Monday, Barcelona midfielder Andres Iniesta said Neymar is coming "to the best place to grow," and that he expects him to become even better when paired in attack with Messi, the four-time world player of the year.
"He can bring us a lot because he is very talented player," Iniesta said. "He brings his ability to break down defenses, to score, and to entertain. He is young. He has a long path ahead of him, and he is coming to the best place to grow."
Iniesta also tried to ease the pressure on his new teammate and allay any concerns fans might have about whether two major stars like Messi and Neymar can play together.
"He is coming to the best possible place for him," Iniesta said. "Let him do what he does best, that is enough. It is great news that we can count on him. Surrounded by the people he will have here, he will get better and better.
"Great players always understand one another. Leo will be make Neymar better, and Neymar will make Leo better."
Dani Alves recalled the long line of Brazilian forwards who have triumphed at Barcelona, including Romario, Ronaldo, Rivaldo and Ronaldinho.
"He will give us a Brazilian touch," countryman Dani Alves said of Neymar. "The others left their mark. I am sure he will, too."
Another question is whether Neymar's arrival will mean the exit of Spain striker David Villa, who also plays on the left flank of Barcelona's attack.
Neymar flew to the Mediterranean city on a private jet with a small entourage, including his father Neymar Silva Santos, that left Rio de Janeiro after the national side's 2-2 draw with England on Sunday. He will return to his home country for the upcoming Confederations Cup, which Brazil is hosting in preparation for the 2014 World Cup.
Neymar led Santos on its greatest run since Pele stopped playing for the Brazilian club in the 1970s. He helped Santos win the 2010 Brazilian Cup, the 2011 Copa Libertadores and three straight Sao Paulo state championships, becoming the team's leading scorer in the post-Pele era with 138 goals in 230 matches.
In 2011, Neymar met many of the members of his new team when Barcelona beat Santos 4-0 in the final of Club World Cup.
Barcelona has not disclosed the financial details of the transfer, but Brazilian media said the Catalan club will pay about 30 million euros for the youngster.
Santos had fielded offers from Barcelona and Madrid and said it was up to Neymar to choose where he wanted to continue his career. Neymar signed an agreement to join the Spanish champions on May 27.