Clubs throughout Europe were put on alert when it emerged Song would be allowed to leave Barcelona - with West Ham having won the race to take him to Upton Park on a temporary basis.
Song revealed it was a conversation with Hammers boss Sam Allardyce that led to his decision to move to east London and he is targeting European qualification.
"When I spoke to the manager, he gave me very good advice," he told West Ham's official website. "I had a very good chat with him and that's why I am coming here. We have to do better than last season. I think the club has very good ambition and I hope we can maybe catch the European positions, because that would be very good for the club.
That's why I chose to come here. I always said that if I was to leave (Barcelona) one day I would return to the Premier League.
Alex Song, West Ham United midfielder
"It was a very hard decision when you have options of a lot of clubs who are playing in the Champions League, but at the end of the day I chose West Ham because I wanted to come to the Premier League."
Song, who won one Spanish La Liga title during his spell in Spain, is likely to be West Ham's final summer signing - taking the total of new recruits to eight. Carl Jenkinson has also joined on loan while Aaron Cresswell, Cheikhou Kouyate, Enner Valencia, Diafra Sakho, Diego Poyet and Mauro Zarate have been signed on permanent deals.
Allardyce said he hoped to complete one more deal before Monday's transfer deadline and, with the acquisition of Song, it would be seen as a surprise if any other new faces came in.
Song was sent off in Cameroon's 4-0 FIFA World Cup™ defeat to Croatia in his last competitive game and could make his debut in the Premier League trip to Hull after the international break, following in the footsteps of his cousin Rigobert Song, who turned out for the Hammers between 2000 and 2002.