Lower Saxony borders on (from north and clockwise) the North Sea, the states of Schleswig-Holstein, Hamburg, Mecklenburg-Western Pomerania, Brandenburg, Saxony-Anhalt, Thuringia, Hesse and North Rhine-Westphalia, and the Kingdom of the Netherlands. The state of Bremen forms two enclaves within Lower Saxony. The state's principal cities include Hanover, Braunschweig (Brunswick), Osnabrück, Oldenburg, and Göttingen.

The northwestern portion of Lower Saxony is a part of Frisia; it is called Ostfriesland (East Frisia) and lies on the coast of the North Sea. It includes seven islands, known as the East Frisian Islands. In the southwest of Lower Saxony is the Emsland, a sparsely populated area, once full of inaccessible swamps. The northern half of Lower Saxony is absolutely flat, but there are two mountain chains in the south: the Weserbergland ("Weser Hilly Region") and the Harz. The middle of the state houses the largest cities and the economic centres: Hanover, Hildesheim, Wolfsburg, Salzgitter and Braunschweig. The region in the northeast is called Lüneburger Heide (Lüneburg Heath), the largest heath of Germany and in medieval times wealthy due to the salt trade. To the north the Elbe river separates Lower Saxony from Hamburg, Schleswig-Holstein, Mecklenburg-Western Pomerania and Brandenburg. The area on the southern banks is called Altes Land, and it is characterised by thousands of fruit trees.