Viktor Fischer has ended his time at Middlesbrough after just one season.
The winger, who joined the Teesiders from Ajax last summer, did not score a goal in his 13 appearances last season.
The Denmark international joins Mainz on a four-year deal.
Fischer told his new club's website: "Mainz is the right club for me to enjoy my football again.
"Mainz is a beautiful city, we have a great stadium, I want to be a part of this team and this club quickly. I want to get in shape and then bring my qualities as an offensive player."
Source: tribalfootball.com