The 23-year-old graduated from the famous La Masia academy seven years ago, but completed a move to Valencia in 2007.
He returned to the Catalans this summer, and he has thanked the fans for their support after returning "home".
"I am delighted to be here with all of you, and also to have returned to my home, which is Barcelona. I want to thank everyone for their great support. The club today remains the best in the world," he told reporters.
"I am proud to be the small kid who made the breakthrough at La Masia, which is bearing so much fruit today. I will give my all for this club."
Alba has 11 Spanish caps to his name.