Mestizo (; Spanish: [mesˈtiso] (listen); fem. mestiza) is a term used for ethno-racial classification to refer to a person of mixed European and Indigenous American ancestry. In certain regions such as Latin America, it may also refer to people who are culturally European even though their ancestors are not. The term was used as an ethno-racial exonym for mixed-race castas that evolved during the Spanish Empire. Although, broadly speaking, mestizo means someone of mixed European/Indigenous heritage, the term did not have a fixed meaning in the colonial period. It was a formal label for individuals in official documents, such as censuses, parish registers, Inquisition trials, and others. Priests and royal officials might have classified persons as mestizos, but individuals also used the term in self-identification.

The noun mestizaje, derived from the adjective mestizo, is a term for racial mixing that did not come into usage until the twentieth century; it was not a colonial-era term. In the modern era, mestizaje is used by scholars such as Gloria Anzaldúa as a synonym for miscegenation, but with positive connotations.

In the modern era, particularly in Latin America, mestizo has become more of a cultural term, with the term indio being reserved exclusively for people who have maintained a separate Indigenous ethnic and cultural identity, language, tribal affiliation, community engagement, etc. In late 19th- and early 20th-century Peru, for instance, mestizaje denoted those peoples with evidence of Euro-indigenous ethno-racial "descent" and access—usually monetary access, but not always—to secondary educational institutions. Similarly, well before the twentieth century, Euramerican "descent" did not necessarily denote Iberian American ancestry or solely Spanish American ancestry (distinct Portuguese administrative classification: mestiço), especially in Andean regions re-infrastructured by Euramerican "modernities" and buffeted by mining labor practices. This conception changed by the 1920s, especially after the national advancement and cultural economics of indigenismo.

To avoid confusion with the original usage of the term mestizo, mixed people started to be referred to collectively as castas. In some Latin American countries, such as Mexico, the concept of the Mestizo became central to the formation of a new independent identity that was neither wholly Spanish nor wholly Indigenous. The word mestizo acquired another meaning in the 1930 census, being used by the government to refer to all Mexicans who did not speak Indigenous languages regardless of ancestry.

During the colonial era of Mexico, the category Mestizo was used rather flexibly to register births in local parishes and its use did not follow any strict genealogical pattern. With Mexican independence, in academic circles created by the "mestizaje" or "Cosmic Race" ideology, scholars asserted that Mestizos are the result of the mixing of all the races. After the Mexican Revolution the government, in its attempts to create an unified Mexican identity with no racial distinctions, adopted and actively promoted the "mestizaje" ideology.