Pierre Clastres (French: [pjɛʁ klastʁ]; 17 May 1934 – 29 July 1977) was a French anthropologist, ethnographer, and ethnologist. He is best known for his contributions to the field of political anthropology, with his fieldwork among the Guayaki in Paraguay and his theory of stateless societies. He mostly researched Indigenous peoples of the Americas in which the power was not considered coercive and chieftains were powerless.

With a background in literature and philosophy, Clastres started studying anthropology with Claude Lévi-Strauss and Alfred Métraux in the 1950s. Between 1963 and 1974 he traveled five times to South America to do fieldwork among the Guaraní, the Chulupi, and the Yanomami. Clastres mostly published essays and, because of his premature death, his work was unfinished and scattered. His signature work is the essay collection Society Against the State (1974) and his bibliography also includes Chronicle of the Guayaki Indians (1972), Le Grand Parler (1974), and Archeology of Violence (1980).

Clastres was born on 17 May 1934, in Paris, France. He studied at the Sorbonne, obtaining a licence in Literature in 1957, and a Diplôme d'études supérieures spécialisées in Philosophy the following year. He began working in Anthropology after 1956 as a student of Claude Lévi-Strauss, working at the Laboratory of Social Anthropology of the French National Centre for Scientific Research during the 1960s. He was also a student of Alfred Métraux at the École pratique des hautes études (EPHE) in 1959.

Clastres's first published article was released in 1962, a year before Clastres went into an eight-month trip to a Guayaki community in Paraguay with the help of Métraux. The Guayaki's study served as base to an article for Journal de la Société des Américanistes, to his 1965 doctoral thesis in ethnology—Social Life of a Nomadic Tribe: The Guayaki Indians of Paraguay—, to "The Bow and the Basket", as well as to his first book, Chronicle of the Guayaki Indians (1972).

In 1965 Clastres returned to Paraguay and he met the Guaraní—this encounter led him to write Le Grand Parler (1974). In 1966 and 1968 Clastres went into expeditions to the Gran Chaco region of Paraguay, where he studied groups of Chulupi people. This experience was used to produce the essays What Makes Indians Laugh and Sorrows of the Savage Warrior. In his fourth expedition Clastres travelled to Venezuela, where he observed the Yanomami people from 1970 to 1971, and wrote The Last Frontier. He briefly visited the Guaraní which migrated from Paraguay to Brazil in his last expedition in 1974.

In 1971 he became lecturer at the fifth section of the EPHE, and was promoted to director of studies of the religion and societies of South American Indigenous peoples in October 1975. That same year he left his office as researcher of the Laboratory of Social Anthropology—which he occupied since 1961—after conflicts over Lévi-Strauss's theories. In 1977 he took in part in the establishment of the journal Libre alongside the former members of Socialisme ou Barbarie Miguel Abensour, Cornelius Castoriadis, Marcel Gauchet, Claude Lefort, and Maurice Luciani. Later that year, Clastres, aged 43, died in Gabriac, Lozère, on 29 July, in a car accident.

Clastres's first book was originally published in France by Plon in 1972 under the title Chronique des indiens Guayaki: ce que que savent les Aché, chasseurs nomades du Paraguay (Chronicle of the Guayaki Indians: The Knowledge of the Aché Hunter Nomads of Paraguay). He was interested in Guayaki because there was little research on them since Alfredo Stroessner's dictatorship forced them to live under territorial restriction and launched a pacifying campaign between 1959 and 1962. In the book, the author describes Guayaki culture with a focus on their cycle of life and their "daily struggles for survival." He describes their mores on rites of passage, marriage, hunting, warfare, and death, as well as their relation with non-Indian people and nature. In 1976 Paul Auster, then a "penniless unknown", translated the book into English but it was only published in 1998 by Zone Books. Auster translated the work because he was fascinated by Clastres's prose, which "seemed to combine a poet's temperament with a philosopher's depth of mind."

Although its literary qualities have been what attracted Auster, the work has been criticized as "romantic". Anthropologist Clifford Geertz said Clastres had a "Rousseauian primitivism, the view that 'savages' are radically different from us, more authentic than us, morally superior to us, and need only to be protected, presumably by us, from our greed and cruelty." Bartholomew Dean, writing for the journal Anthropology Today, declared, "Clastres' ahistoricism, rhetorical romanticism, and museumification sadly obscures the ongoing challenges facing indigenous peoples like the Guayaki."