Brazil striker Neymar completed his move from Santos to Barcelona for £48.6m (57m euros) on Monday.
The 21-year-old, who agreed to join the Spanish champions in May, passed his medical and signed a five-year deal.
He said: "Money is OK but happiness takes priority. We decided to come to Barcelona. I had a lot of offers but I followed my heart.
"I think adjusting to European football may be difficult but I hope to adjust quickly."
The player - who was a target for Real Madrid - is to travel back to Brazil for the upcoming Confederations Cup.
After arriving in Catalonia on a specially chartered private jet on Monday afternoon, hundreds of photographers were waiting to take his picture as he posed in front of the club's emblem.
Neymar - who has scored 20 goals in 33 appearances for his country - then passed a two-part medical examination and put pen to paper on a deal which runs until 2018.
He was later presented on the Nou Camp pitch in front of tens of thousands of screaming supporters.
"The thrill of being cheered when I entered the Nou Camp - it was hard not to cry," added the Brazilian. "My dream has come true.
"I've never worried about being the best in the world. The best is already here and that's Messi.
"I'm one of the luckiest men in the world to be able to play with him and it's an honour. I'm very happy to be realising my life's dream.
"To have the opportunity to play with great players I admire like Messi, Xavi and Andres Iniesta - I've begun a new stage of my life and I'm going to be very happy and achieve a lot."
The signing ends years of speculation about which team would be the first to persuade him to make the move to Europe.
He led the club to their greatest run since Brazil legend Pele stopped playing for the club in the 1970s, helping them win the 2010 Brazilian Cup, the 2011 Copa Libertadores and three straight Sao Paulo state championships.
He is the club's leading scorer in the post-Pele era with 138 goals in 229 matches.
Neymar is also considered Brazil's key player for the 2014 World Cup, which they are hosting. Next on his agenda is the warm-up event for the tournament, the Confederations Cup, which runs from 15-30 June.