Unethical human experimentation is human experimentation that violates the principles of medical ethics. Such practices have included denying patients the right to informed consent, using pseudoscientific frameworks such as race science, and torturing people under the guise of research. Around World War II, Imperial Japan and Nazi Germany carried out brutal experiments on prisoners and civilians through groups like Unit 731 or individuals like Josef Mengele; the Nuremberg Code was developed after the war in response to the Nazi experiments. Countries have carried out brutal experiments on marginalized populations. Examples include American abuses during Project MKUltra and the Tuskegee syphilis experiments, and the mistreatment of indigenous populations in Canada and Australia. The Declaration of Helsinki, developed by the World Medical Association, is widely regarded as the cornerstone document on human research ethics.

Nazi Germany performed human experimentation on large numbers of prisoners (including children), largely Jews from across Europe, but also Romani, Sinti, ethnic Poles, Soviet prisoners of war, homosexuals and disabled Germans, in its concentration camps mainly in the early 1940s, during World War II and the Holocaust. Prisoners were forced into participating; they did not willingly volunteer and no consent was given for the procedures. Typically, the experiments resulted in death, trauma, illness, shortening of life, disfigurement, or permanent disability, and as such are considered as examples of medical torture since the participants had to endure mass amounts of pain.

At Auschwitz and other German camps, under the direction of Eduard Wirths, selected inmates were subjected to various hazardous experiments that were designed to help German military personnel in combat situations, develop new weapons, aid in the recovery of military personnel who had been injured, and to advance the racial ideology backed by the Third Reich. Aribert Heim conducted similar medical experiments at Mauthausen. At Buchenwald, Carl Værnet conducted experiments on homosexual prisoners in an attempt to "cure" homosexuality.

After the war, these crimes were tried at what became known as the Doctors' Trial, and the abuses perpetrated led to the development of the Nuremberg Code of medical ethics. During the Nuremberg Trials, 23 Nazi doctors and scientists were tried for the unethical treatment of concentration camp inmates, who were often used as research subjects with fatal consequences. Of those 23, 16 were convicted (15 were convicted for the unethical treatment, while one of them was only convicted of SS membership), 7 were condemned to death, 9 received prison sentences from 10 years to life, and 7 were acquitted.

The Law for the Prevention of Genetically Defective Progeny, passed on 14 July 1933, legalized the involuntary sterilization of persons with diseases claimed to be hereditary: weak-mindedness, schizophrenia, alcohol abuse, insanity, blindness, deafness, and physical deformities. The law was used to encourage growth of the Aryan race through the sterilization of persons who fell under the quota of being genetically defective. 1% of citizens between the age of 17 to 24 had been sterilized within two years of the law passing. Within four years, 300,000 patients had been sterilized. From about March 1941 to about January 1945, sterilization experiments were conducted at Auschwitz, Ravensbrück, and other places by Carl Clauberg. The purpose of these experiments was to develop a method of sterilization which would be suitable for sterilizing millions of people with a minimum of time and effort. These experiments were conducted by means of X-ray, surgery and various drugs. Thousands of victims were sterilized. Aside from its experimentation, the Nazi government sterilized around 400,000 people as part of its compulsory sterilization program. Intravenous injections of solutions speculated to contain iodine and silver nitrate were successful, but had unwanted side effects such as vaginal bleeding, severe abdominal pain, and cervical cancer. Therefore, radiation treatment became the favored choice of sterilization. Specific amounts of exposure to radiation destroyed a person's ability to produce ova or sperm. The radiation was administered through deception. Prisoners were brought into a room and asked to complete forms, which took two to three minutes. In this time, the radiation treatment was administered and, unknown to the prisoners, they were rendered completely sterile. Many suffered severe radiation burns.

Eugen Fischer began sterilization experimentation in German-occupied South West Africa during World War I. A supporter of forced sterilization as a means to prevent the growth of inferior populations and a member of the Nazi Party, Fischer focused his experimentation on mixed-race children in order to justify the Nazi Party's ban on interracial marriage. As a result of Fischer's research in Namibia, Germany prohibited marriages between people of different races in its colonies.

The Luftwaffe performed a series of 360 to 400 experiments at Dachau and Auschwitz, in which hypothermia was induced in 280 to 300 victims. These were conducted for the Nazi high command to simulate the conditions the armies suffered on the Eastern Front, as the German forces were ill-prepared for the cold weather they encountered. Many experiments were conducted on captured Russian troops; the Nazis wondered whether their genetics gave them superior resistance to cold. Approximately 100 people are reported to have died as a result of these experiments.

In early 1942, prisoners at Dachau concentration camp were used by Sigmund Rascher in experiments to aid German pilots who had to eject at high altitudes. A low-pressure chamber containing these prisoners was used to simulate conditions at altitudes of up to 20,000 m (66,000 ft). Of the 200 subjects, 80 died outright, and the others were executed.