It lies in southern Niedersachsen in the foothills of the Harz and Solling Mountains. On the north are the heights of the Helleberg, on the southwest the Hube, and on the northwest the Selter. The Gande, a northeastern tributary of the Leine, flows through the center of Kreiensen.
For most of the 20th century, Kreiensen was known as both a railroad hub (with five lines interchanging, for example Hanoverian Southern Railway) and an important logistic center for the Federal German Mail (Deutsche Bundespost). When the latter was privatized and the new high-speed rail line (Hannover-Würzburg) was opened in 1991, Kreiensen quickly lost a number of jobs in both sectors.