The Single Convention on Narcotic Drugs, 1961 (Single Convention, 1961 Convention, or C61) is an international treaty that controls activities (production, supply, trade, use) of specific narcotic drugs and establishes a system of regulations (licenses, measures for treatment, research, etc.) for their medical and scientific uses.

The treaty updated the Paris Convention of 13 July 1931, to include the vast number of synthetic opioids invented in the intervening 30 years and to add a mechanism for more easily including new ones. From 1931 to 1961, most of the families of synthetic opioids had been developed, including drugs related to methadone, pethidine (meperidine/Demerol), morphinans, and dextromoramide (Palfium, Palphium, Jetrium, Dimorlin, marketed solely in the Netherlands). Research on fentanyls and piritramide (R-3365, Pirium, Dipidolor, Piridolan, among others) was also nearing fruition at that point.

Earlier treaties had only controlled opium, coca, and derivatives such as morphine, heroin, and cocaine. The Single Convention, adopted in 1961, consolidated those treaties and broadened their scope to include cannabis and other substances with effects similar to drugs already covered. The Commission on Narcotic Drugs and the World Health Organization were empowered to add, remove, and transfer drugs among the treaty's four schedules of controlled substances. The International Narcotics Control Board was put in overall control of drug production, international trade, and dispensation. The United Nations Office on Drugs and Crime (UNODC) was delegated the Board's daily monitoring of each country and working with national authorities to ensure compliance with the Single Convention. This treaty has since been supplemented by the Convention on Psychotropic Substances, which controls LSD, MDMA, and other psychoactive pharmaceuticals, and the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances, which strengthens provisions against money laundering and other drug-related offenses.

As of February 2018, the Single Convention has 186 state parties. The Holy See, the State of Palestine plus all member states of the United Nations are state parties, with the exception of Chad, East Timor, Equatorial Guinea, Kiribati, Nauru, Samoa, South Sudan, Tuvalu, and Vanuatu.