The 29-year-old, who has developed into one of Mainz's most important players over the past few seasons, had reportedly reached a basic agreement with RB Leipzig over a move before deciding against it.
"There's been a lot written about me over the last few days. There have been a lot of rumours, and I've also had a lot of offers," Amiri said, according to Sky Sports.
"But today my family and I decided that I will definitely be staying here."
Mainz sporting director Christian Heidel welcomed the decision.
"I'm really pleased for us. Nadiem is a great guy, and this is simply the right match," Heidel said. "To be honest, though, I never expected anything other than for him to stay."
Heidel had already made clear in recent weeks that Mainz felt no pressure to sell key players this summer, also naming Kaishu Sano and Paul Nebel among those the club had no intention of letting go without receiving significant value in return.
Amiri joined Mainz from Bayer Leverkusen in 2024 and has made 82 appearances for the club, scoring 26 goals and adding 13 assists. A Germany international with two appearances at this summer's World Cup, he is contracted at Mainz until the summer of 2028. Leipzig's interest had been growing since March 2025, with reports suggesting a fee in the region of €15-20m was being discussed before Amiri's decision brought the speculation to an end.
Amiri, who is considered one of the most accomplished footballers of Afghan heritage given his standing in the German game, made his announcement at a traditional fan evening during Mainz's pre-season training camp, with the club's supporters greeting the news with loud celebration.
It is not the first time he has used the occasion to settle transfer talk: last year, similar speculation over a Leipzig move was also resolved in Mainz's favour at the same event, reinforcing the sense that Amiri's long-term future lies with the Rheinhessen for now.