A prison uniform is a set of standardized clothing worn by prisoners. It usually includes visually distinct clothes worn to indicate the wearer is a prisoner, in clear distinction from civil clothing.

Prison uniforms are intended to make prisoners instantly identifiable, limit risks through concealed objects and prevent injuries through undesignated clothing objects. A prison uniform can also spoil attempts of escape, as prison uniforms typically use a design and color scheme that is easily noticed and identified even at a greater distance. Wearing a prison uniform is typically done only reluctantly and is often perceived as stigmatizing, and as an invasion into the autonomy of decision.

The United Nations Standard Minimum Rules for the Treatment of Prisoners (The Mandela Rules) first adopted in 1955 and amended in 2015, prohibit degrading or humiliating clothing, requiring in Rule 19 that:

During the Victorian era when prison sentences of prolonged durance were implemented in the judicial system of several countries, actual garments were conceived to be worn specifically by prison inmates, which developed to the various types of prison uniforms presently in use.

All incarcerated people in Romania wear their own clothes. Until around 2007, when Romania joined the EU, prison uniforms existed, but there is few sources on whether they were compulsory for all people behind bars, compulsory for all convicts, used by some detainees, used by some convicts, or used by both some detainees and convicts. It is however known that a short time after the collapse of communism (in 1992) uniforms were compulsory for all people behind bars. It is known that the Aiud prison required inmates to wear khaki uniforms and inmates serving a life sentence wore orange uniforms. As for other prisons, many probably also used khaki uniforms as human rights website shows inmates in khaki uniforms at Rahova prison.

While prison uniforms are used for both convicts and detainees, they are not compulsory. The policies are similar to those in Germany. Some prisons do not use them at all, some prisons use them only for some inmates (depending on conviction of crime or behavior), and some prisons make uniforms compulsory. Minimum security prisons are encouraged to not use prison uniforms and maximum security prisons are encouraged to use them, however this all varies as some inmates incarcerated for small crimes do wear uniforms and some inmates incarcerated for serious crimes do not wear them. Uniforms are almost always worn on top of civilian clothes. Usually on all security levels the uniform is a green button-down jacket worn on top of civilian clothes with green jacket pants but some inmates in maximum security prisons and non-serious criminals who are badly misbehaved sometimes wear orange or red instead of green.

During the Nazi period of Germany, interned people in the concentration camp system were often made to wear prisoner's uniforms. In today's Germany, inmates may wear regular civilian clothing in some prisons. In other prisons clothing issued by the prison is compulsory. If a prisoner cannot afford to have his own clothing cleaned and/or replaced, they may be issued with clothing. There are also facilities with no prison uniforms.

The prison uniforms are officially referred to as Anstaltskleidung (literally: "institutional clothing"), not as "uniforms". They are usually similar to the type of clothing generally worn for manual work, and not necessarily recognizable as prison clothing. When prisoners are allowed to temporarily leave prison, they may generally wear private clothing to avoid being recognized as prisoners.