A developed country (or industrialized country, high-income country, more economically developed country (MEDC), advanced country) is a sovereign state that has a high quality of life, developed economy, and advanced technological infrastructure relative to other less industrialized nations. Most commonly, the criteria for evaluating the degree of economic development are the gross domestic product (GDP), gross national product (GNP), the per capita income, level of industrialization, amount of widespread infrastructure and general standard of living. Which criteria are to be used and which countries can be classified as being developed are subjects of debate. Different definitions of developed countries are provided by the International Monetary Fund and the World Bank; moreover, HDI ranking is used to reflect the composite index of life expectancy, education, and income per capita. Another commonly used measure of a developed country is the threshold of GDP (PPP) per capita of at least USD$22,000. In 2022, 36 countries fit all four criteria, while an additional 17 countries fit three out of four.

Developed countries have generally more advanced post-industrial economies, meaning the service sector provides more wealth than the industrial sector. They are contrasted with developing countries, which are in the process of industrialisation or are pre-industrial and almost entirely agrarian, some of which might fall into the category of Least Developed Countries. As of 2015, advanced economies comprise 60.8% of global GDP based on nominal values and 42.9% of global GDP based on purchasing-power parity (PPP) according to the IMF.