The Denmark international moves to the Bundesliga side on a four-year deal after making just six starts in the Premier League
Viktor Fischer has left Middlesbrough after just one season to join Bundesliga side Mainz on a four-year deal.
The Denmark international moved to the Riverside from Ajax, where he forged a reputation as an exciting winger and won two Eredivisie titles during four seasons in the Dutch capital.
However, Fischer endured a disappointing spell in England and racked up just six Premier League starts as the club were relegated straight back to the Championship.
Fischer has consequently opted for a new direction in his career and links up with Mainz, who finished 15th in the Bundesliga last term, until 2021.