The 25-year-old, who has spent his entire career with Freiburg, joins for an undisclosed fee following the Baden-Wurttemberg-based club’s relegation to the 2. Bundesliga.
“I’m delighted with this move to Hannover,” Sorg said. “I think this is the right step for me.
“The club, its environment and the city of Hannover are totally in keeping with what I am looking for.”
Sorg picked up his first cap for Germany in 2014 having come through Freiburg’s youth system to make his Bundesliga debut in 2011.
He has gone on to make 105 Bundesliga appearances, scoring three goals.
“He’s a versatile defender who, with his experience, his attitude and his character, will fit into our team excellently,” said Hannover’s director of sport Dirk Dufner.