Peter Ulrich Weiss (8 November 1916 – 10 May 1982) was a German writer, painter, graphic artist, and experimental filmmaker of adopted Swedish nationality. He is particularly known for his plays Marat/Sade and The Investigation and his novel The Aesthetics of Resistance.

Peter Weiss earned his reputation in the post-war German literary world as the proponent of an avant-garde, meticulously descriptive writing, as an exponent of autobiographical prose, and also as a politically engaged dramatist. He gained international success with Marat/Sade, the American production of which was awarded a Tony Award and its subsequent film adaptation directed by Peter Brook. His "Auschwitz Oratorium" The Investigation, served to broaden the debates over the so-called "Aufarbeitung der Vergangenheit" (or formerly) "Vergangenheitsbewältigung" or "politics of history". Weiss's magnum opus was The Aesthetics of Resistance, called one of the "most important German-language work[s] of the 70s and 80s." His early, surrealist-inspired work as a painter and experimental filmmaker remains less well known.

Weiss was born in Nowawes (now part of Potsdam-Babelsberg), near Berlin, to a Hungarian Jewish father from Nagy Emöke, Nyitra County (whose parents were Moric Weisz and Fanny Steiner from Szered near Pozsony) and a Christian mother from Switzerland.

After the First World War and the break-up of the Austro-Hungarian Empire Weiss's father became a Czech citizen and the son acquired his father's citizenship – Weiss was never a German citizen. Jenö Weisz converted to Protestanism in 1920. Peter Weiss was unaware of his father's Jewish origin until the 1930s. At the age of three, he moved with his family to the German port city of Bremen, and during his adolescence back to Berlin, where he began training as a painter. In 1935, he emigrated with his family to Chislehurst, near London, England, where he studied photography at the Polytechnic School of Photography. In 1936–1937 the family moved to Czechoslovakia. Weiss attended the Prague Art Academy. After the German occupation of the Czech Sudetenland in 1938, his family moved to Sweden, while Weiss was visiting Hermann Hesse in Switzerland. In 1939 he joined his family in Stockholm, Sweden, where he lived for the rest of his life. He became a Swedish citizen in 1946. Weiss was married three times: to the painter Helga Henschen, 1943–47; to Carlota Dethorey, 1949; and from 1964 until his death to the Swedish artist and stage designer Gunilla Palmstierna-Weiss.

In the 1960s, Weiss became increasingly politically radical, taking stands for revolutionary Cuba and against US intervention in Vietnam and visiting both countries. In 1966, he visited the United States together with the West German writers group Gruppe 47. During a conference at Princeton University, he denounced the US war against North Vietnam which seems to have scandalized his German colleagues more than his US hosts. In 1967, he participated in the anti-war Russell Tribunal in Stockholm and in 1968 he joined the eurocommunist Swedish Left Party (VPK). During the same year, he also visited North Vietnam and published a book about his trip.

In 1970, Weiss suffered a heart attack. During the following decade, he wrote his monumental three-part novel, The Aesthetics of Resistance, as well as two very different stage versions of Kafka's novel The Trial. He died in Stockholm in 1982.

During his early life as a painter – 1930 to 1950 – Weiss was influenced by old Dutch masters such as Pieter Breughel, and Hieronymus Bosch. After World War II his painting, as well as his work in film and literature, came under the lasting influence of Surrealism. He taught painting at Stockholm's People's University, and illustrated a Swedish edition of The Book of One Thousand and One Nights. In 1952, he joined the Swedish Experimental Film Studio, where he directed several experimental short films, followed by several socially conscious documentary shorts: Gesichter im Schatten (Faces in the Shadow, 1956), Im Namen des Gesetzes (In the Name of the Law, 1957), Was machen wir jetzt? (What Do We Do Now?, 1958). In 1959, he directed his only full-length (experimental) film Hägringen (The Disappeared). In the early 1950s, Weiss had begun to write again, producing a number of prose works, some in German, others in Swedish. Most are short, intense, surrealist text which suggest the influence of Kafka (whose work Weiss would later adapt for the stage).

The most important of these prose texts is Der Schatten des Körpers des Kutschers (The Shadow of the Body of the Coachman, 1952). It is a nearly hermetic experimental work that explores language through the use of surreal, disturbing imagery whereby an apparent rural idyll is transformed into a kafkaesque nightmare. The Surrealist effect was enhanced by collages in the style of Max Ernst, so-called xylography, which Peter Weiss created for the book. Coachman has been linked both to the French nouveau roman of Alain Robbe-Grillet and Raymond Queneau, as well as to French absurdist works by Samuel Beckett, Eugene Ionesco, and Jean Genet. When it was eventually published in Germany in 1960 it put its 45-year-old writer at the forefront of the West German literary scene. Weiss abandoned painting and filmmaking and turned exclusively to writing. Nearly all his subsequent works – and all of the major ones – are written in German. His next prose work, Abschied von den Eltern (Leavetaking, 1959/60), was less hermetic than Coachman and strongly autobiographical. It was not only a critical but also a public success, as was its follow-up, Fluchtpunkt (Vanishing Point, 1962).