Song, 27, joined the Hammers on a season-long loan deal in August 2014.
And the Hammers have again landed the former Arsenal player on loan from the Spanish giants.
Gold tweeted: “It has just been confirmed. Alex Song has signed a season long loan deal to join West Ham. Welcome back Alex.”
Cameroon-born Song made 31 appearances for West Ham in all competitions last season.
Earlier this year he announced his international retirement, citing a wish to focus on rebuilding his domestic career with Hammers.
Song, who joined Barcelona for £15million from Arsenal in August 2012, made 65 appearances for the Catalan side prior to his first loan switch to the Hammers in 2014.
He becomes West Ham’s eighth arrival during the transfer window, joining goalkeeper Darren Randolph, defenders Angelo Ogbonna and Carl Jenkinson and midfielders Pedro Obiang, Dimitri Payet, Manuel Lanzini and George Dobson at Slaven Bilic’s team.
West Ham later completed the season-long loan signing of Chelsea winger Victor Moses.