Sachsenhausen (German pronunciation: [zaksn̩ˈhaʊzn̩] ^ⓘ) or Sachsenhausen-Oranienburg was a Nazi concentration camp in Oranienburg, Germany, used from 1936 until April 1945, shortly before the defeat of Nazi Germany in May. It mainly held political prisoners throughout World War II. Prominent prisoners included Joseph Stalin's oldest son, Yakov Dzhugashvili; assassin Herschel Grynszpan; Paul Reynaud, the penultimate prime minister of the French Third Republic; Francisco Largo Caballero, prime minister of the Second Spanish Republic during the Spanish Civil War; the wife and children of the crown prince of Bavaria; Ukrainian nationalist leader Stepan Bandera; and several enemy soldiers and political dissidents.

Sachsenhausen was a labour camp, outfitted with several subcamps, a gas chamber, and a medical experimentation area. Prisoners were treated inhumanely, fed inadequately, and killed openly. After World War II, when Oranienburg was in the Soviet Occupation Zone, the structure was used by the NKVD as NKVD special camp Nr. 7. Today, Sachsenhausen is open to the public as a memorial.

The camp was finished in 1936. It was located 35 kilometres (22 mi) north of Berlin, which gave it a primary position among the German concentration camps: the administrative centre of all concentration camps was located in Oranienburg, and Sachsenhausen became a training centre for Schutzstaffel (SS) officers (who would often be sent to oversee other camps afterwards). Initially, the camp was used to perfect the most efficient and effective execution method for use in the death camps. Given that, executions obviously took place at Sachsenhausen, especially of Soviet prisoners of war.

During the earlier stages of the camp's existence, the executions were done by placing the prisoners in a small room, often even with music playing, called the Genickschussbaracke (English: Shot in the neck barrack) and told they were to have their height and weight measured but were instead shot in the back of the neck through a sliding door located behind the neck. This was found to be far too time-consuming, so they then trialled a trench, killing either by shooting or by hanging. While this more easily enabled group executions, it created too much initial panic among the prisoners, making them harder to control. Then small scale trials of what would go on to become the large scale, death camp gas chambers were designed and carried out. These trials showed the authorities that this method facilitated the means to murder the largest number of prisoners without "excessive" initial panic. So by September 1941, when they were conducting the first trials of this method at Auschwitz, Sachsenhausen had already been the scene of "some gassings in conjunction with the development of gas vans".

The prisoners were also used as a workforce, with a large task force of prisoners from the camp sent to work in the nearby brickworks to meet Albert Speer's vision of rebuilding Berlin.

In July 1936, the Esterwegen concentration camp and Columbia concentration camp were closed and those prisoners moved to the Oranienburg concentration camp. That summer, those prisoners began clearing an 80 hectares (200 acres) of triangular forested area. By 1937, the prisoners had erected prisoners' barracks and SS guards' quarters, and SS officers' families housing. The "protective custody" internment camp was laid out in an isosceles triangle with sides 600 m (2,000 ft) long. Tower A was at its central control point, linked to the SS troop camp outside along the central axis. The entire camp could be viewed by the SS command staff from Tower A. Initially 18 hectares (44 acres) in area, the camp eventually grew to cover 400 hectares (990 acres). Designed by Bernhard Kuiper, Himmler called Sachsenhausen a "completely new concentration camp for the modern age, which can be extended at any time." In practice, however, extending the design proved impractical.

There was an infirmary inside the southern angle of the perimeter and a camp prison within the eastern angle. There was also a camp kitchen and a camp laundry. The camp's capacity became inadequate and the camp was expanded in 1938 by a new rectangular area (the "small camp") northeast of the entrance gate and the perimeter wall was altered to enclose it. There was an additional area (Sonderlager) outside the main camp perimeter to the north; this consisted of two huts Sonderlager 'A' and 'B' built in 1941 for special prisoners that the regime wished to isolate.

The neutral zone was located between the camp wall and the prisoners' camp. Between the zone and the wall was a trip wire, cheval de frise, barbed-wire obstacles, an electrified barbed-wire fence, and a sentry path.