Sanllehi will leave the Nou Camp after 14 years when his contract expires on 1 December and officially begins his duties in North London in February, the club have confirmed.
His role at the Emirates will see him "lead future player negotiations" as part of a restructured model at the club, working alongside manager Arsene Wenger, chief executive Ivan Gazidis and chief contract negotiator Huss Fahmy.
His arrival follows that of Sven Mislintat, the highly-regarded scout who has been named the club's new head of recruitment operations after calling time on a hugely successful spell withBorussia Dortmund.
Sanllehi has played a central role in conducting transfers at Barcelona across the last decade and was especially active in the recruitment of players from South America – most significantly the deal that saw the Catalans beat Real Madrid to the signing of Neymar in 2013.
Manchester City were also linked with the deal-maker and were said to have held an advantage of their Premier League rivals in recruiting him, given Sanllehi had previously worked with City chiefs Ferran Soriano and Txiki Begiristain at Barcelona. But it is Arsenal who have now confirmed the services of the notorious "transfer closer".
His arrival in February means Arsenal are unlikely to immediately benefit from his deep reach in the transfer world when the January window opens, but it is another promising addition going forward as the club continues a significant restructure off the pitch.
Gazidis said: "In the past three years we have supplemented our team with more top-class expertise across every aspect of our football operations.
"Raul's appointment is another important step in developing the infrastructure we need at the club to take everything we do to the next level. Raul has extensive contacts across the football world and has been directly involved in some of the biggest transfers in Europe in recent years. We look forward to him bringing that expertise to Arsenal."