The 26-year-old Cameroon international struggled to hold down a regular place at the Nou Camp following a move from Arsenal in 2012 and returns to the Premier League with the Hammers.
He was paraded at Upton Park ahead of the clash with Southampton on Saturday afternoon.
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 sign him on a temporary basis despite reported interest from the likes of Napoli and Galatasaray.
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 whufc.com.
"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.
"West Ham is looking to be a very, very big club in the future. The club has a great project and it is a very big project with very big ambition.
"I think it's great for the fans that the club wants to be one of the biggest in England. 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.
"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 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.