Neymar chose Barcelona over their Spanish arch rivals Real Madrid after Santos ended three years of resisting offers for the 21-year-old forward and let him go before next year's tournament in Brazil.
"On Monday I'm signing a contract with Barcelona," Neymar said on his Instagram page and Twitter account.
Santos, on their Facebook page, and Barcelona on their website confirmed Neymar's decision.
"My family and friends already know about my decision... I want to thank Santos' fans for these nine incredible years. My feelings for the club and the fans will never change," Neymar added.
"Titles, goals, dribbles, celebrations and the songs the fans created for me will always be in my heart.
"This is a different moment for me, sad [departure] and happy [new challenge. God bless me in my choices.
"I made a point of playing tomorrow's match in Brasilia. I want the opportunity to enter the pitch once more with the 'mantle' [Santos shirt] and hear the fans shout my name."
Santos play Flamengo at the capital's completely refurbished World Cup stadium in their opening match of the Brazilian championship on Sunday.
Barcelona said Neymar "stands out for his polished skills, he makes a difference and scores goals... he has received many accolades and the praise of the celebrated Pele, of whom he is considered his heir."
However, since the middle of last year a move had become increasingly difficult for Neymar to resist while Santos lost the form that took them to their third South American Libertadores Cup title in 2011 and first since Pele's 1960s heyday.
"After rejecting a series of million-dollar offers for Neymar Jr, since the middle of 2010... against the trend in Brazilian football of selling its best t
