Polyethnicity, also known as pluri-ethnicity or multi-ethnicity, refers to specific cultural phenomena that are characterized by social proximity and mutual interaction of people from different ethnic backgrounds, within a country or other specific geographic region.

Same terms may also relate to the ability and willingness of individuals to identify themselves with multiple ethnicities. It occurs when multiple ethnicities inhabit a given area, specifically through means of immigration, intermarriage, trade, conquest and post-war land-divisions. This has had many political and social implications on countries and regions.

Many, if not all, countries have some degree of polyethnicity, with countries like Nigeria and Canada having high levels and countries like Japan and Poland having very low levels (and more specifically, a sense of homogeneity). The amount of polyethnicity prevalent in some Western countries has spurred some arguments against it, which include a belief that it leads to the weakening of each society's strengths, and also a belief that political-ethnic issues in countries with polyethnic populations are better handled with different laws for certain ethnicities.

In 1985, Canadian historian William H. McNeill gave a series of three lectures on polyethnicity in ancient and modern cultures at the University of Toronto. The main thesis throughout his lectures was the argument that it has been the cultural norm for societies to be composed of different ethnic groups. McNeill argued that the ideal of homogeneous societies may have grown between 1750 and 1920 in Western Europe because of the growth in the belief in a single nationalistic base for the political organization of society. McNeill believed that during World War I, the desire for homogeneous nations began to weaken.

Polyethnicity divides nations, complicating the politics as local and national governments attempt to satisfy all ethnic groups. Many politicians in countries attempt to find the balance between ethnic identities within their country and the identity of the nation as a whole. Nationalism also plays a large part in these political debates, as cultural pluralism and consociationalism are the democratic alternatives to nationalism for the polyethnic state.

The idea of nationalism being social instead of ethnic entails a variety of culture, a shared sense of identity and a community not based on descent. Culturally-plural states vary constitutionally between a decentralized and unitary state (such as the United Kingdom) and a federal state (such as Belgium, Switzerland, and Canada). Ethnic parties in these polyethnic regions are not anti-state but instead seek maximum power within this state. Many polyethnic countries face that dilemma with their policy decisions. The following nations and regions are just a few specific examples of this dilemma and its effects:

The United States is a nation founded by different ethnicities frequently described as coming together in a "melting pot," a term used to emphasize the degree to which constituent groups influence and are influenced by each other, or a "salad bowl," a term more recently coined in contrast to the "melting pot" metaphor and emphasizing those groups' retention of fundamentally distinct identities despite their proximity to each other and their influence on the overall culture that all of those groups inhabit.

A controversial political issue in recent years has been the question of bilingualism. Many immigrants have come from Hispanic America, who are native Spanish speakers, in the past centuries and have become a significant minority and even a majority in many areas of the Southwest. In New Mexico the Spanish speaking population exceeds 40%. Disputes have emerged over language policy, since a sizeable part of the population, and in many areas the majority of the population, speak Spanish as a native language.