The 24-year-old joins for an undisclosed fee and has penned a four-and-a-half year deal at the Coface Arena, bringing an end to a three-year stint at Wolfsburg.
Koo - who has made 10 league appearances thus far this season - boasts 12 goals in 35 appearances for South Korea.
"He is a very tough, versatile player with exceptional technical qualities and a high work rate," said Mainz coach Thomas Tuchel.
"His way of playing football and his character make him a very good addition to our team. I'm very happy that we've managed to complete this transfer, as he can give our team an extra boost."
Koo was loaned out to Augsburg from January 2012 until the end of the following season, scoring eight times.
He began his career at K-League outfit Jeju United before making the switch to Germany.
Koo's departure from Wolfsburg is offset by the arrival of attacking midfielder Kevin De Bruyne, who completed his move from Premier League Chelsea on Saturday.
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