According to Italian daily Corriere Fiorentino, West Ham are working on a deal to sign the 32-year-old defensive midfielder and are close to getting a deal over the line.
West Ham have been looking for a new striker to bolster their attacking options and now look set to land Perez on a permanent deal.
West Ham director of football Mario Husillos added: "He is a very experienced player that Manuel and I both know well from Spain".
Despite only securing his switch to the Olympic Stadium today, Perez could be a part of the West Ham squad for their opening Premier League fixture this weekend.
Edmilson Fernandes could be on his way to Italy as part of a deal taking Carlos Sanchez away from Fiorentina, as the Serie A side are keen on landing the Switzerland global as makeweight in a deal - and have until the end of the month to find an agreement.
Sanchez spent second half of last season on loan at Espanyol and his contract with Fiorentina expires at the end of next season.
Perez, who has signed a three-year deal, spent just two years at the Emirates Stadium - one of those out on loan at Deportivo - and scored seven goals in 21 appearances.
"Listen, it's great first and foremost to come back and see them, but I'm a West Ham player".