The Hammers have handed the former Deportivo La Coruna ace a three-year deal at the London Stadium, taking their summer spending close to the £100m mark.
Perez has endured a frustrating two years at Arsenal since arriving in a £17m deal, having been restricted to two Premier League starts in his first campaign and then sent out on loan to Deportivo in 2017-18.
Speaking to his new side's official website for the first time, the Spaniard admitted that he is pleased to have joined West Ham at an exciting time.
"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 added: "We welcome Lucas to West Ham United. He is a very experienced player that Manuel [Pellegrini] 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."