Fictive kinship (less often, fictional kinship) is a term used by anthropologists and ethnographers to describe forms of kinship or social ties that are based on neither consanguineal (blood ties) nor affinal ("by marriage") ties. It contrasts with true kinship ties.

To the extent that consanguineal and affinal kinship ties might be considered real or true kinship, the term fictive kinship has in the past been used to refer to those kinship ties that are fictional, in the sense of not-real. Invoking the concept as a cross-culturally valid anthropological category therefore rests on the presumption that the inverse category of "(true) kinship" built around consanguinity and affinity is similarly cross-culturally valid. Use of the term was common until the mid-to-late twentieth century, when anthropology effectively deconstructed and revised many of the concepts and categories around the study of kinship and social ties. In particular, anthropologists established that a consanguinity basis for kinship ties is not universal across cultures, and that—on the contrary—it may be a culturally specific symbol of kinship only in particular cultures (see the articles on kinship and David M. Schneider for more information on the history of kinship studies).

Stemming from anthropology's early connections to legal studies, the term fictive kinship may also be used in a legal sense, and this use continues in societies where these categories and definitions regarding kinship and social ties have legal currency; e.g. in matters of inheritance.

As part of the deconstruction of kinship mentioned above, anthropologists now recognize that—cross-culturally—the kinds of social ties and relationships formerly treated under the category of "kinship" are often not predicated on blood ties or marriage ties, and may rather be based on shared residence, shared economic ties, nurture kinship, or familiarity via other forms of interaction.

In sociology of the family, this idea is referred to as chosen kin, fictive kin or voluntary kin. Sociologists define the concept as a form of extended family members who are not related by either blood or marriage. The bonds allowing for chosen kinship may include religious rituals, close friendship ties, or other essential reciprocal social or economic relationships. Examples of chosen kin include godparents, adopted children, and close family friends. The idea of fictive kin has been used to analyze aging, foreign fighters, immigrant communities, and minorities in modern societies. Some researchers state that peers have the potential to create fictive kin networks.

Types of relations often described by anthropologists as fictive kinship include compadrazgo relations, foster care, common membership in a unilineal descent group, and legal adoption. A noted Gurung tradition is the institution of "Rodi", where teenagers form fictive kinship bonds and become Rodi members to socialize, perform communal tasks, and find marriage partners. In Western culture, a person may refer to close friends of one's parents as "aunt" or "uncle" (and their children as "cousin"), or may refer to close friends as "brother" or "sister", although this is just a mere courtesy treatment and does not represent an actual valuation as such. In particular, college fraternities and sororities in some North American cultures usually use "brother" and "sister" to refer to members of the organization. Monastic, Masonic, and Lodge organisations also use the term "Brother" for members. "Nursing Sister" is used to denote a rank of nurse, and the term "Sisterhood" may be used for feminists. Fictive kinship was discussed by Jenny White in her work on female migrant workers in Istanbul. In her work, she draws on ideas of production and the women she works with being drawn together through "webs of indebtedness" through which the women refer to each other as kin. These relationships are, however, less frequent than kin relationships, and serve purposes that are neither comparable to nor exclude a natural family.

Recently, many anthropologists have abandoned a distinction between "real" and "fictive" kin, because many cultures do not base their notion of kinship on genealogical relations. This was argued most forcefully by David M. Schneider, in his 1984 book A critique of the study of kinship. In response to this insight, Janet Carsten developed the idea of "relatedness". She developed her initial ideas from studies with the Malays in looking at what was socialized and biological. Here she uses the idea of relatedness to move away from a pre-constructed analytics opposition which exists in anthropological thought between the biological and the social. Carsten argued that relatedness should be described in terms of indigenous statements and practices, some of which fall outside what anthropologists have conventionally understood as kinship.

This does not imply, however, that human non-kin relationships, such as in tit-for-tat situations, even within a friendship relation, are more important than kin relationships, since their motivation is also related to one's survival and perpetuation, or that people are necessarily bound to the culture they are inserted in, nor can it be generalized to the point of claiming all individuals always undervalue kinship in the absence of nurturing. In those cases, attachment to others is not a cultural act but an act of survival. Herbert Gintis, in his review of the book Sex at Dawn, critiques the idea that human males were unconcerned with parentage, "which would make us unlike any other species I can think of". Such individuals can be considered out of the natural tendency of living beings for survival through offspring.