Didac Vila completed his move to the San Siro on Friday.
"I am very happy to be here and ready for the challenges that await me," he told the official Milan website.
"I hope the fans will grow to love me the same way the Espanyol supporters did. I am a left-back, but feel able to play both in a defensive and offensive role.
"I think that I have joined the best club in Europe and the side with the most trophies in the world."