Borussia Dortmund have strengthened their defensive options ahead of the transfer deadline with the acquisition of Park Joo-ho from Mainz.

The South Korea international arrives at Signal Iduna Park on a three-year contract, having made 42 Bundesliga starts in two seasons for Mainz since arriving from Basel.

He will go head-to-head with Marcel Schmelzer for a regular starting spot in Thomas Tuchel's side.

"Park Joo-ho is the ideal addition to our squad at left-back," sporting director Michael Zorc told the club's official website.

Meanwhile, Mainz wished their former player well after learning of his desire to make the switch to the ever-improving Dortmund.

"Park Joo-ho approached us with the absolute desire of switching to Borussia Dortmund," said manager Christian Heidel. 

"After we were able to reach an agreement with Borussia Dortmund, we did not obstruct him this way, especially since we have found a good solution for the position of left-back.

"The two years together with Park stay with us very good memories. We wish him well in his new club every success."