Denmark international midfielder Viktor Fischer has become the fourth major signing to arrive at Mainz this summer.
The 23-year-old joins the 05ers from Middlesbrough on a four-year deal through to 30 June 2021.
"I was completely swayed by what sporting director Rouven Schröder and coach Sandro Schwarz had to say about Mainz 05 and their vision for the club," Fischer told Mainz's official club website. "I want to become a part of this team and this club as quickly as possible, find my form as quickly as possible and then showcase my qualities as an attacking player."
A 15-time Denmark international, Fischer made 16 Premier League appearances for Middlesbrough last season. Prior to that, he spent four years on the books at Ajax, registering 39 goals and 19 assists in 111 appearances for Dutch football's biggest club.
"Viktor Fischer is a very exciting player with huge potential," commented Mainz sporting director Rouven Schröder. "He's also a technically gifted, skillful and clinical attacker, who will enhance our squad and make us more flexible. We're delighted he's decided his future is at Mainz 05."
In what has been a busy start to the summer transfer window, Mainz have already acquired the services of former Germany goalkeeper Rene Adler, Romania midfielder Alexandru Maxim and Bosnia-Herzegovina forward Kenan Kodro, as well as promoting Under-23s boss Sandro Schwarz to the position of first-team coach.
Mainz defender Alexander Hack also signed a contract extension, valid until 2022, on Wednesday.
The 05ers finished 15th last season, avoiding a place in the relegation play-off on goal difference.
Watch: Mainz ensured their survival with a game to spare in 2016/17