The player first confirmed the news on social networking sites before the club confirmed the impending deal on their website, and revealed the deal would come with a five-year contract.
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.
"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," he added.
"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.
The club said on Friday they had agreed to transfer Neymar, letting him choose between two offers.
Folha de Sao Paulo newspaper put the latest offer from Barcelona at 74 million Brazilian Real (£23.88 million).
Neymar, regarded as former Santos and Brazil great Pele's heir, will play alongside Argentine ace Lionel Messi, FIFA World Player of the Year for the last four years, at Barcelona.
The Spanish champions are looking to revamp their stellar squad after what by their high standards is a slump during which they have been eliminated from the European Champions League at the semi-final stage for the last two seasons.
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