Goal revealed on Tuesday that the 29-year-old was heading to the London Stadium after reaching an agreement with the Hammers.
A move has now been pushed through for a man who struggled to make the desired impact at Arsenal following a £17m ($22m) switch in 2016.
He has now decided to trade north London for the East End, with Lucas joining Ryan Fredericks, Lukasz Fabianski, Issa Diop, Jack Wilshere, Aindriy Yarmolenko, Fabian Balbuena and Felipe Anderson in an elaborate West Ham rebuild under Manuel Pellegrini.
“I’m very happy to be here at such an historic club,” he told the Hammers’ official website.
“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 of the forward's arrival: “We welcome Lucas to West Ham United.
“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.”