Friedrich "Fritz" Hartjenstein (3 July 1905 – 20 October 1954) was an SS-Obersturmbannführer (lieutenant colonel) in the SS-Totenkopfverbände. He worked at various Nazi concentration camps such as Auschwitz and Sachsenhausen. After the Second World War, he was tried for war crimes and found guilty for murder and crimes against humanity.
Hartjenstein, who was born in Peine, began his SS work at Sachsenhausen in 1938. The following year he was transferred to Niederhagen. In 1941 Hartjenstein served for a year with the 3rd SS Division Totenkopf, a Waffen SS combat division.
In 1942, he was appointed the commandant of Birkenau. This was the main camp at Auschwitz, which contained the extermination facilities and crematoria. In 1944 Hartjenstein was appointed commandant of Natzweiler concentration camp in France. In 1945 he went to work at Flossenbürg concentration camp.
He was arrested by the British and sentenced to life imprisonment on 6 June 1946 at Wuppertal for executing four French Resistance members. Hartjenstein was then tried by the British for hanging a Royal Air Force POW. He was sentenced to death by firing squad.