The 31-year-old is now committed to the Reds until 2015, while he will automatically receive a further one-year extension based on appearances.
Pillar of the side
"We're delighted that Leon will continue being a pillar of our team," said Hannover's sporting director Dirk Dufner. "He shows plenty of passion and 100 per cent effort on the field and he was an important stabilising influence in many games for us in the defensive midfield."
Andreasen has become Hannover's longest-serving active player having recently broken through the 100-appearance barrier in the Bundesliga. "It wasn't hard for me to commit to Hannover 96," said the Dane, who joined from Fulham FC in 2009. "I identify fully with the club and hope that we will have more success together."