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 sixth signing of the summer for new boss Slaven Bilic.
He follows Darren Randolph, Pedro Obiang, Stephen Hendrie, Dimitri Payet, Angelo Ogbonna and Carl Jenkinson into the club.