Hannover coach Mirko Slomka told his club's official website: "Emanuel Pogatetz is an experienced player who can also play at left-back.
"He strengthens our options in the defensive areas."
Pogatetz leaves Teesside after a five-year spell, having made 158 appearances for the club, and was part of the side that reaches the Uefa Cup final in 2006, although he missed out on the final through injury.
Prior to his move to England, Pogatetz enjoyed spells with AK Graz, Bayer Leverkusen and FC Aarau.
Hannover have also signed Karlsruhe goalkeeper Markus Miller.
Source: BBC Sport
Source: BBC Sport