Koo, who has made ten appearances for the Wolves this season, will sign a four-and-a-half-year contract with der Karnevalverein after completing his medical on Saturday.
"A very good addition"
"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."
The 24-year-old midfielder has scored twelve goals in 35 appearances for South Korea. He also made 36 league appearances for FC Augsburg during an 18-month loan spell between 2012 and 2013, scoring eight times.