The 29-year-old joins Ryan Fredericks, Lukasz Fabianski, Issa Diop, Jack Wilshere, Aindriy Yarmolenko, Fabian Balbuena and Felipe Anderson in moving to the London Stadium this summer.
News of Perez’s arrival was leaked by Jack Wilshere, who posted a video of Perez in the dressing room on Instagram before West Ham had the chance to unveil it.
Twitter insider @ExWHUemployee, a reliable source of Hammers transfer news, now believes more will follow on a dramatic day at the London Stadium.
He tweeted: “I would imagine he is the first announcement on what should be quite a busy day both with ins and outs.”
Perez, 29, joined the Gunners two years ago but spent the entirety of last season on loan at former club Deportivo.
Having been included in Arsenal's pre-season tour of Singapore it appeared Perez was potentially going to be given a fresh start under new head coach Unai Emery.
But a transfer across London to join the Hammers has been done, with Perez wanting another chance to make his mark in England.
“I’m very happy to be here at such an historic Club,” he said.
“I’m very excited, as West Ham are giving me the chance to stay in the Premier League and I’m very happy to be part of this new project.
“West Ham have great fans and play in an amazing stadium, so to sign here brings me great joy. I’m very motivated.”
Director of football Mario Husillos welcomed Lucas to West Ham and believes Manuel Pellegrini’s squad will be the perfect environment for the Spaniard to thrive.
“We welcome Lucas to West Ham United,” he said. “He is a very experienced player that Manuel and I both know well from Spain.
“He has great technical quality, and can play as both the main striker or the second striker in a deeper role. He scores goals, but can also create them and link well with the midfielders.
“His style is different from the other strikers we have available to us, and that gives the manager more options for our attacking play.
“Lucas also has experience of the Premier League with Arsenal, and is familiar with life here in London, which of course means that he will take less time to adapt and be ready.
“From the very first meeting I had with Lucas, he made it very clear that he wanted to come to West Ham United and, for us, this is very important. We wish him every success during his time here.”