The Barcelona director is set to leave the Nou Camp on December 1 and take up his new role at the Emirates in February.
Sanllehi’s imminent arrival follows the appointment former Borussia Dortmund chief scout Sven Mislintat.
Arsenal say Sanllehi will operate alongside Arsene Wenger and Ivan Gizadis to oversee future player negotiations.
Chief executive Gazidis said: “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 has been credited with playing a key role in Barcelona’s signing of Neymar from Brazilian club Santos and has a reputation as one of Europe’s most respected transfer negotiators.
He will not be in position for the upcoming transfer window, but is expected to play a large role in Arsenal’s summer dealings.