Former River Plate and Fluminense player Lanzini, 22, joins from United Arab Emirates club Al-Jazira.
Gold wrote on Twitter: “Manuel Lanzini an attacking midfielder has today signed for West Ham United. He’s nicknamed ‘the jewel’.”
The diminutive Lanzini, who scored eight goals in 24 games for Al-Jazira last season, is the seventh signing of the summer for new boss Slaven Bilic.
He follow Darren Randolph, Pedro Obiang, Stephen Hendrie, Dimitri Payet, Angelo Ogbonna and Carl Jenkinson into the club.
“This is very emotional because it is one of the best teams I have ever signed for,” he told West Ham TV. “It is a very important step for my career and for my future.
“I am looking forward to what is a new era for me. I’m very happy and I look forward to meeting my new team-mates.
“This is a new challenge for me and that can only help in the future. It will test my ability in one of the best leagues in the world, so I am looking forward to getting started.
“I am someone who always gives 100 per cent. I’m an attacking player, I can change the game’s tempo and I hope to give my maximum for the fans to appreciate me.”