Lower Saxony (German: Niedersachsen [ˈniːdɐzaksn̩] (listen); Low German: Neddersassen; Saterland Frisian: Läichsaksen) is a German state (Landcode: deu promoted to code: de ) in northwestern Germany. It is the second-largest state by land area, with 47,614 km² (18,384 sq mi), and fourth-largest in population (8 million in 2021) among the 16 Ländercode: deu promoted to code: de federated as the Federal Republic of Germany. In rural areas, Northern Low Saxon and Saterland Frisian are still spoken, albeit in declining numbers.

Lower Saxony borders on (from north and clockwise) the North Sea, the states of Schleswig-Holstein, Hamburg, Mecklenburg-Vorpommerncode: deu promoted to code: de , Brandenburg, Saxony-Anhalt, Thuringia, Hesse and North Rhine-Westphalia, and the Netherlands. Furthermore, the state of Bremen forms two enclaves within Lower Saxony, one being the city of Bremen, the other its seaport, Bremerhaven (which is a semi-enclave, as it has a coastline). Lower Saxony thus borders more neighbours than any other single Bundeslandcode: deu promoted to code: de . The state's largest cities are state capital Hanover, Braunschweig (Brunswick), Lüneburg, Osnabrück, Oldenburg, Hildesheim, Salzgitter, Wolfsburgcode: deu promoted to code: de , and Göttingen.

Lower Saxony is the only Bundeslandcode: deu promoted to code: de that encompasses both maritime and mountainous areas. The northwestern area of the state, on the coast of the North Sea, is called East Frisia and the seven East Frisian Islands offshore are popular with tourists. In the extreme west of Lower Saxony is the Emsland, an economically emerging but rather sparsely populated area, once dominated by inaccessible swamps. The northern half of Lower Saxony, also known as the North German Plain, is almost invariably flat except for the gentle hills around the Bremen geestland. Towards the south and southwest lie the northern parts of the Central Uplands: the Weser Uplands and the Harz Mountains. Between these two lie the Lower Saxon Hills, a range of low ridges.

Lower Saxony's major cities and economic centres are mainly situated in its central and southern parts, namely Hanover, Braunschweig, Osnabrück, Wolfsburgcode: deu promoted to code: de , Salzgitter, Hildesheim, and Göttingen. Oldenburg, near the northwestern coastline, is another economic centre. The region in the northeast, the Lüneburg Heath (Lüneburger Heidecode: deu promoted to code: de ), is the largest heathland area of Germany. In the Middle Ages, it was wealthy due to salt-mining and the salt trade, as well as, to a lesser degree, the exploitation of its peat bogs, which went on until the 1960s. To the north the Elbe River separates Lower Saxony from Hamburg, Schleswig-Holstein, Mecklenburg-Vorpommerncode: deu promoted to code: de , and Brandenburg. The banks just south of the Elbe are known as the Altes Landcode: deu promoted to code: de (Old Country). Due to its gentle local climate and fertile soil, it is the state's largest area of fruit farming, its chief produce being apples.

Most of the state's territory was part of the historic Kingdom of Hanover, and the state of Lower Saxony has adopted the coat of arms and other symbols of the former kingdom. It was created by the merger of the State of Hanover with three smaller states on 1 November 1946.