The 23-year-old will sign a permanent deal at Newcastle subject to passing a medical examination, as Manquillo's current contract with Atletico Madrid is set to expire next summer.
The right-back is an ex-Marseille and Liverpool loanee and spent last season on loan at the Magpies' north-east rivals Sunderland, where he made 20 league appearances under David Moyes.
Manquillo would become Benitez’s third major summer signing, with Newcastle having already completed the signings of Florian Lejuene from Eibar and Christian Atsu, who made his loan move permanent.