Sanllehi, who spent 14 years with Barcelona, will join the Premier League side in February and will be in charge of "future player negotiations" while working alongside manager Arsene Wenger.
Arsenal chief executive Ivan 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."
Sanllehi helped pave the way for one of the most successful periods in Barcelona's history with his role in helping the team lure some of the world's most talented football players, such as Neymar and Luis Suarez.
His arrival is part of a reshuffle of Arsenal's backroom staff, which has seen the club also announce the arrival of Sven Mislintat from Borussia Dortmund to be the team's new head of player recruitment operations.