The deal is set to be confirmed on either Thursday or Friday, with the Japanese international set to become the first arrival at the club since the appointment of Unai Emery last month.
The Japanese teenager starred this campaign on a season-long loan deal at Real Mallorca, scoring four goals and providing five assists across 36 appearances – including 24 starts.
There were reports earlier this month that Real Sociedad and Sevilla were among the clubs hopeful of securing a loan deal for the player next season.
Indeed, the latest report claims there were upwards of five La Liga sides in the mix to land the highly-rated player this month, but the Yellow Submarine have won the race to his signature although the deal will not have any option to sign him permanently.
The attack-minded Kubo spent four years in the Blaugrana’s La Masia youth academy but had to leave the club in 2015 when Barcelona were sanctioned for signing underage players illegally.
Kubo, who has been nicknamed ‘the Japanese Messi’, then signed for FC Tokyo but he celebrated his 18th birthday last summer and was subsequently available to return to European football – with Madrid pouncing.