The 24-year-old midfielder, who joined Wolfsburg from Korean outfit Jeju United in 2011, had only played 10 times this season in all competitions and with the signing of Belgian international Kevin de Bruyne on Saturday from Chelsea his chances of more playing time were vastly reduced.

Both clubs refused to reveal the transfer fee.

"Koo Ja-Cheol has been on our wish list for quite some time now and we intensified our efforts of late as we consider him being a potentially key player in our team," said Mainz manager Christian Heidel.

Mainz coach Thomas Tuchel highlighted the 'talent and technical qualities' of Koo.

Wolfsburg had already loaned Koo out to Augsburg last season before returning to them for this campaign.

De Bruyne, 22, signed a five-and-a-half year contract with Wolfsburg on Saturday for a reported club record fee of 22 million euros ($29.8m, �18.1m).