Decriminalization or decriminalisation is the reclassification in law relating to certain acts or aspects of such to the effect that they are no longer considered a crime, including the removal of criminal penalties in relation to them. This reform is sometimes applied retroactively but otherwise comes into force from either the enactment of the law or from a specified date. In some cases regulated permits or fines may still apply (for contrast, see: legalization), and associated aspects of the original criminalized act may remain or become specifically classified as crimes. The term was coined by anthropologist Jennifer James to express sex workers' movements' "goals of removing laws used to target prostitutes", although it is now commonly applied to drug policies. The reverse process is criminalization.

Decriminalization reflects changing social and moral views. A society may come to the view that an act is not harmful, should no longer be criminalised, or is otherwise not a matter to be addressed by the criminal justice system. Examples of subject matter which have been the subject of changing views on criminality over time in various societies and countries include:

• abortion
• breastfeeding in public
• drug possession, and recreational drug use
• euthanasia
• gambling
• homosexuality
• polygamy
• prostitution
• public nudity
• steroid use in sport
• suicide

In a federal country, acts may be decriminalized by one level of government while still subject to penalties levied by another; for example, possession of a decriminalized drug may still be subject to criminal charges by one level of government, but another may yet impose a monetary fine. This should be contrasted with legalization, which removes all or most legal detriments from a previously illegal act. It has also been noted that while some acts have been decriminalised, such as homosexuality and adultery, others have increased in their criminalization, such as incest.