Slomka will now remain at the helm of the Lower Saxony side until 2016, ending months of negotiations.
"The contractual foundations have been laid for the successful collaboration to continue," read a statement on the club's website.
The paperwork was signed on Saturday after a verbal agreement was reached in midweek.
Slomka has led Hannover to their most successful years, qualifying twice for the Europa League and reaching the quarter-finals of the competition last season.
He took over with the club at the foot of the Bundesliga and was able to guide them to safety in his first few months at the helm before taking them to fourth in his first full year, only just missing out on a place in the Champions League behind Bayern Munich.
Talks over a new contract had been ongoing for a year, culminating in Saturday's agreement.