The Olympic silver medalist has joined for £5million, after the two clubs reached an agreement over a deal.
He passed his medical on Wednesday morning before being unveiled at the Weserstadion on the afternoon.
“I had very good talks with the management and that has led me here to Werder and the Bundesliga.
“The Olympics were a personal highlight for me and I'd like to carry on my development here at Werder.
“I've come to Bremen to get playing time, to develop further and to help the team."