The 32-year-old was a free agent after deciding not to extend his contract at Hamburg beyond the 2016/17 campaign.
"The Mainz transfer is the right step at the right time for me," Adler explained. "The calm and focused atmosphere at the club suits me. In a sporting sense, I have the feeling I can achieve something exciting here. And the lengths [sporting director] Rouven Schröder and [head coach] Sandro Schwarz went to during the talks to sign me really impressed me."
Leipzig-born Adler, who spent five years at Hamburg, has been a regular fixture in the Bundesliga since breaking onto the scene with Bayer Leverkusen midway through the 2006/07 season.
He has since made 255 appearances in the German top flight, and was part of the Germany squad that finished runners-up to Spain at UEFA EURO 2008.
Watch: Rene Adler was the hero for Hamburg against Gladbach on Matchday 7
"Rene Adler has been among the Bundesliga's biggest personalities of the century," affirmed Mainz sporting director Rouven Schröder. "He's also been one of the best goalkeepers in Germany for years. Both factors make him a very exciting player for Mainz. We quickly struck up a good rapport during the talks. We got to know him as a highly motivated professional, who fits perfectly into the Mainz 05 project."