Organized crime (or organised crime) is a category of transnational, national, or local groupings of highly centralized enterprises run by criminals to engage in illegal activity, most commonly for profit. While organized crime is generally thought of as a form of illegal business, some criminal organizations, such as terrorist groups, rebel forces, and separatists, are politically motivated. Many criminal organizations rely on fear or terror to achieve their goals or aims as well as to maintain control within the organization and may adopt tactics commonly used by authoritarian regimes to maintain power. Some forms of organized crime simply exist to cater towards demand of illegal goods in a state or to facilitate trade of goods and services that may have been banned by a state (such as illegal drugs or firearms). Sometimes, criminal organizations force people to do business with them, such as when a gang extorts money from shopkeepers for "protection". Street gangs may often be deemed organized crime groups or, under stricter definitions of organized crime, may become disciplined enough to be considered organized. A criminal organization can also be referred to as a gang, mafia, mob, (crime) ring, or syndicate; the network, subculture, and community of criminals involved in organized crime may be referred to as the underworld or gangland. Sociologists sometimes specifically distinguish a "mafia" as a type of organized crime group that specializes in the supply of extra-legal protection and quasi-law enforcement. Academic studies of the original "Mafia", the Italian Mafia, which predates the other groups, generated an economic study of organized crime groups and exerted great influence on studies of the Russian mafia, the Chinese Triads, the Hong Kong Triads, and the Japanese Yakuza.

Other organizations—including states, churches, militaries, police forces, and corporations—may sometimes use organized-crime methods to conduct their activities, but their powers derive from their status as formal social institutions. There is a tendency to distinguish "traditional" organized crime from certain other forms of crime that also usually involve organized or group criminal acts, such as white-collar crime, financial crimes, political crimes, war crimes, state crimes, and treason. This distinction is not always apparent and academics continue to debate the matter. For example, in failed states that can no longer perform basic functions such as education, security, or governance (usually due to fractious violence or to extreme poverty), organized crime, governance, and war sometimes complement each other. The term "oligarchy" has been used to describe democratic countries whose political, social, and economic institutions come under the control of a few families and business oligarchs that may be deemed or may devolve into organized crime groups in practice. By their very nature, kleptocracies, mafia states, narco-states or narcokleptocracies, and states with high levels of clientelism and political corruption are either heavily involved with organized crime or tend to foster organized crime within their own governments.

In the United States, the Organized Crime Control Act (1970) defines organized crime as "[t]he unlawful activities of [...] a highly organized, disciplined association [...]". Criminal activity as a structured process is referred to as racketeering. In the UK, police estimate that organized crime involves up to 38,000 people operating in 6,000 various groups. Historically, the largest organized crime force in the United States has been La Cosa Nostra (Italian-American Mafia), but other transnational criminal organizations have also risen in prominence in recent decades. A 2012 article in a U.S. Department of Justice journal stated that: "Since the end of the Cold War, organized crime groups from Russia, China, Italy, Nigeria, and Japan have increased their international presence and worldwide networks or have become involved in more transnational criminal activities. Most of the world's major international organized crime groups are present in the United States." The US Drug Enforcement Administration's 2017 National Drug Threat Assessment classified Mexican transnational criminal organizations (TCOs) as the "greatest criminal drug threat to the United States," citing their dominance "over large regions in Mexico used for the cultivation, production, importation, and transportation of illicit drugs" and identifying the Sinaloa, Jalisco New Generation, Juárez, Gulf, Los Zetas, and Beltrán-Leyva cartels as the six Mexican TCO with the greatest influence in drug trafficking to the United States. The United Nations Sustainable Development Goal 16 has a target to combat all forms of organised crime as part of the 2030 Agenda.

In some countries, football hooligans can be considered a criminal organization if it engages in illicit and violent activities.