Werder Bremen pounce for Papastathopoulos
"We've been watching him for a long time and all the information we have collected has been very convincing," coach Thomas Schaaf told the club's website.
"He is dogged and aggressive in a positive way, is good in the air and likes to join in the attack."
Werder said the 23-year-old, who has 18 caps and played at the World Cup last year, will wear the number 22 shirt which belonged to former captain Torsten Frings.
"That's a big motivation for me, that such an important player has worn this number," said Papastathopoulos, who spent last season at AC Milan where he made only five Serie A appearances.