Sanllehi is the second backroom change this month after the appointment of Sven Mislintat as head of recruitment.
Sanllehi will join the club in February after 14 years at Barcelona.
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.”
The changes to Arsenal’s backroom team point to new visions for the club – which could put Arsene Wenger’s position in to some doubt.
Wenger signed up for two more years in the summer but has admitted his position will be reviewed in the summer.