The 19 year-old moves to Barca from Defensor Sporting and can play at centre-half and left-back.
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Pacifico will initially be registered with Barca Atletic.
The left-back had already been in Barcelona for a few days awaiting the finalisation of the agreement between the two clubs, which was completed on Monday.
"It's a huge honour to wear this shirt; it's a dream every footballer has because Barça is a very big club," Pacifico told the club's media after putting pen to paper.
The deal is for a loan to the end of the season with an option to buy.