The former Arsenal star was presented to supporters at Upton Park before the Hammer's Premier League match against Southampton on Saturday.
As revealed by Goal, the Cameroonian flew into London on Friday to finalise the move, having failed to solidify a first-team place during his two years at Camp Nou.
Manager Sam Allardyce wrote in the matchday programme of completing "one more high-quality signing", with Barcelona reportedly subsidising part of Song's wages for the duration of the spell.
The Cameroon international struggled for first-team minutes at Camp Nou, being afforded just 13 Liga starts last season, and is now expected to replace Mohammed Diame who is being tracked by both Hull and Everton.
Galatasaray were keen to sign the 26-year-old on a permanent deal but the former Arsenal man now becomes West Ham seventh summer signing, joining the likes of Enner Valencia, Mauro Zarate and Cheikhou Kouyate.