A developed country is a country that has achieved (currently or historically) a high degree of industrialization, and which enjoys the higher standards of living which wealth and technology make possible. In most cases, countries with high per capita GDP are "developed countries"; sometimes high GDP can be achieved (usually temporarily) through natural resource exploitation (e.g. Saudi Arabia, oil; Nauru, phosphate) without the country becoming developed.

Observers often see strong correlations between countries with high economic development and their possessing robust democratic institutions or free market economies, though in neither case is the correlation absolute or uncontroversial.

Synonyms include industrialised countries, more economically developed countries (MEDC) and the First World.

Other terms sometimes used to describe the developed/developing country dichotomy are first world/third world (the term second world was during the Cold War reserved for communist states,) North/South, or industrialized countries/non-industrialized countries. The term Western countries has similar, though not identical, connotations.

In the United Nations system there is no established convention for the designation of "developed" and "developing" countries or areas. In common practice, Japan in Asia, Canada and the United States in northern America, Australia and New Zealand in Oceania and Europe are considered "developed" regions or areas. In international trade statistics, the Southern African Customs Union is also treated as a developed region and Israel as a developed country; and countries of eastern Europe and the former U.S.S.R. countries in Europe are not included under either developed or developing regions.

Generally, according to various sources (the World Bank, the OECD, and the United Nations), developed countries include the following:

Note: Countries in italics are countries in which its status as "developed" is disputed among the sources consulted.