The 20-year-old attacker, named the league's best young player of 2018, was set to undergo a medical with the Spanish side and would join up with their 'B' side.
Financial details were not released but earlier reports said Barca would pay around 2 million euros for Abe, who made his senior international debut at the Copa America and appeared in all three of their matches.
“It was a difficult decision to leave (Kashima) mid-season, but I chose to make the move because I absolutely want to take on this new challenge at Barcelona,” Kyodo news agency quoted Abe as saying in a statement.
The club have already had a busy summer, signing Frenkie de Jong from Ajax, Neto from Valencia and Emerson from Atlético Mineiro for a combined 113 million euros.
The Spanish giants also signed signed England Under-16 striker Louie Barry on a three-year contract from West Bromwich Albion yesterday.
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