The Stanford prison experiment (SPE), also referred to as the Zimbardo prison experiment (ZPE), was a controversial psychological experiment performed in August 1971 at Stanford University. It was designed to be a two-week simulation of a prison environment that examined the effects of situational variables on participants' reactions and behaviors. Stanford University psychology professor Philip Zimbardo managed the research team who administered the study. Zimbardo ended the experiment early after realizing the guard participants' abuse of the prisoners had gone too far.

Participants were recruited from the local community through an advertisement in the newspapers offering $15 per day ($116.18 in 2025) to male students who wanted to participate in a "psychological study of prison life". 24 participants were chosen after assessments of psychological stability and then assigned randomly to the role of prisoners or prison guards. Critics have questioned the validity of these methods.

Those volunteers selected to be "guards" were given uniforms designed specifically to de-individuate them, and they were instructed to prevent prisoners from escaping. The experiment started officially when "prisoners" were arrested by real police of Palo Alto. During the next five days, psychological abuse of the prisoners by the "guards" became increasingly brutal. After psychologist Christina Maslach visited to evaluate the conditions, she was troubled to see how study participants were behaving and she confronted Zimbardo. He ended the experiment on the sixth day.

The experiment has been referenced and critiqued as an example of an unethical psychological experiment, and the harm inflicted on the participants in this and other experiments during the post-World War II era prompted American universities to improve their ethical requirements and institutional review for human experiment subjects in order to prevent them from being similarly harmed. Other researchers have found it difficult to reproduce the study, especially given those constraints.

Certain critics have described the study as unscientific and fraudulent. In particular, Thibault Le Texier has established that the guards were asked directly to behave in certain ways in order to confirm Zimbardo's conclusions, which were largely written in advance of the experiment. Zimbardo claimed that Le Texier's article was mostly ad hominem and ignored available data that contradicts his counterarguments, but the original participants, who were interviewed for the National Geographic documentary The Stanford Prison Experiment: Unlocking the Truth, have largely confirmed many of Le Texier's claims.