Song, 26, has spent the last two seasons at the Nou Camp following his £15million move from Arsenal in the summer of 2012, playing 39 La Liga matches and winning the title in 2012/13.
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 despite reported interest from the likes of Napoli and Galatasaray.
The Cameroon international, who can also play at centre-back, is delighted to be returning to London with the Hammers and cannot wait to face the challenge of Premier League football again.
Song said: "I'm very happy to be here and very happy to see the guys and the fans today and to start to do my job for West Ham United.
"I think this is very important for me to be coming back to London to West Ham United, which is a club with ambition to build a new stadium and become a big club. I am very excited to be part of this project.
"For me, when I was in Spain, I always said that if I was to leave one day I would return to the Premier League. It was a very hard decision, because 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. This is a league which has given me a lot and I didn't finish what I wanted to achieve here.
"For my family too, it was very important, because my children and my wife wanted to come back to England. This is an opportunity to do that with West Ham and I just took it. I am very happy to be here and very proud to be part of the project at this club."
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