|Koo Ja-cheol (AP-Yonhap News)|
The transfer was announced on the same day Wolfsburg signed forward Kevin de Bruyne from Chelsea.
Mainz says Koo received a contract valid through mid-2018.
Koo moved to Wolfsburg in January 2011 and spent the second half of last season on loan to Augsburg.
Mainz coach Thomas Tuchel says Koo’s “way of playing football and his character fit very well into our team.”
Ji receives 4-year contract
DORTMUND, Germany (AP) ― Borussia Dortmund says it has signed South Korea forward Ji Dong-won to a four-year contract starting next season.
Ji will play for Bundesliga rival Augsburg until the end of this season, before moving on to Dortmund on a free transfer.
The South Korean completed his move from Sunderland to Augsburg on Thursday, saying he wanted a new club to get more playing time and earn a spot on South Korea’s team for the World Cup in Brazil.
Ji already played for Augsburg in the second half of last season on loan from Sunderland and helped the German team avoid relegation.