The promising midfielder has signed a five-year deal with the Karnevalsverein, with both clubs agreeing to stay silent on any transfer fee.
Öztunali could make his Mainz debut this Saturday, when they travel to the Westfalenstadion to take on Borussia Dortmund in their first Bundesliga game of the season.
Plenty of potential
Martin Schmidt has secured one of the most talented youngsters in Germany, with the attacking midfielder keen to get to work and show why he is so highly thought of.
Öztunali began his career at Hamburger SV and then switched to Bayer Leverkusen, before going out on loan to Werder Bremen over the last two seasons.
He made 41 Bundesliga appearances for the Weserstadion outfit, helping them to narrowly avoid relegation.
Competition for places
While this looks like a great get for Mainz, there are doubts over whether or not Öztunali will start straight away.
One position that Schmidt's side have plenty of cover in is attacking midfield, and he will have to fight his way past several first-team regulars for a spot.
Yunus Malli's place is not in doubt, though Öztunali's bid to jump up the pecking order will be aided by Jairo's MCL injury.
Even then, he'll have to fight his way past Pablo de Blasis, Christian Clemens, Gerrit Holtmann and Karim Onisiwo for a starting berth.
Öztunali has his say
Speaking to the club website after the deal had gone through, the youngster spoke positively about the switch: "Mainz 05 is the right side for me in the next step of my career."
He added, "Their style of play is aggressive and they always play on the front foot which suits me."
Öztunali concluded, "I’d like to get to work here immediately, establish myself in the team and help them out on the pitch.”