Austrian painter, printmaker and photographer. He began painting as a self-taught artist in the mid-1940s, after leaving school, and first came into contact with contemporary art through a British Council exhibition in 1947 that included work by Paul Nash, Francis Bacon, Stanley Spencer, Henry Moore and Edward Burra. Around this time he produced his first portraits, such as Rainer Dying (pencil, 1949). While attending the Staatsgewerbeschule at Villach from 1947 to 1949 he became interested in theories of Surrealism. He had almost no academic training as an artist, leaving the Hochschule für Angewandte Kunst in Vienna in 1949 after only one day because of an argument with a teacher, and lasting little longer at the Akademie der Bildenden Künste in Vienna in 1950. From 1948 to 1951 he produced Surrealistic drawings representing underwater scenes and mystical forms, rendering these fantastic images in pencil as a densely worked surface. In 1950 he produced the first of his prints; over the next 20 years he explored a variety of media, including etching, drypoint, lithography and screenprinting. In works such as Winnetou (pencil, 920×745 mm, 1950) he explored microstructures and sought to expose repressed impulses in defiance of bourgeois hypocrisy. Deeply suspicious of rationality, he investigated the potential of dreams, madness and the subconscious; to these ends he co-founded the Hundsgruppe, whose members also included Ernst Fuchs, under the influence of French Surrealism in 1950.

During a visit to Paris in 1951 he first saw gestural abstractions by painters such as Georges Mathieu, Jackson Pollock, Jean-Paul Riopelle and Wols; the spontaneous marks and physical interaction with the surface characteristic of Art informel and Abstract Expressionism were emulated by him in his subsequent work, although he remained attached to representational subject-matter. Although he met André Breton during this stay in Paris, he began to turn away from fantastic Surrealism. From 1951 to 1954 he worked on a series entitled Blind Drawings (e.g. 1952), in which he studied optical disintegration and the destruction of form, replacing pictorial composition and illusion with the immediacy of accidentally encountered textures. Basing his methods in part on the Surrealist technique of Automatism, he produced paintings such as Atomization (oil on hardboard, 1950–51) and drawings dominated by clusters of strokes and sometimes worked over with coloured chalk crayon. After moving to the countryside at Gainfarn, near Vienna, in 1953, where he remained until 1959, he sought to counter this exploration of irrational and arbitrary impulses by producing works based on objective mathematical principles, beginning with a series of Proportion Studies in 1953–4 (e.g. Proportion Study, 1953). During these same years he produced black-and-white pictures and the first of his many works painted over photographs for which he himself often served as the model (e.g. Dead Self-portrait, 1955). From 1953 to 1965 he devoted himself principally to a series of Overpaintings, in which he obliterated his early expressive drawings or pictures by friends with whose work he was in sympathy, to produce almost monochrome paintings dominated by black or red, for example Red Overpainting (1958). In building up a rough and encrusted relief-like surface that shows the trace of the brush and blobs of paint, he gradually asserted the predominance of the reworked surface over the virtually invisible original below.
From 1956 Rainer became concerned with religious theories and practices, particularly in a group of paintings dominated by cruciform shapes, such as Black Cross (1956; Munich, Lenbachhaus). The interest in extreme emotional states hinted at in such works became even more pronounced in 1963, when he began to collect paintings by the insane, and in 1964, when he experimented with hallucinogenic drugs. From 1968 he used photographs of his hands and often grimacing face as the basis of partly overpainted works, such as Face in Face (1968–70). His concern with the variety of facial expression and from 1969 with the expressiveness of body language was a conscious means of breaking taboos against what is ugly, absurd or instinctual. As with the Austrian performance artists associated with Aktionismus, in deliberately calling forth extreme emotional states Rainer sought to convey a sustained and intense experience to the viewer. Eschewing the ‘good manners' of conventional techniques, from 1973 he also exploited the ‘primitive' and inarticulate energy of finger-painting, as in Finger Smear (1974).

From the mid-1970s Rainer reworked photographs on a variety of subjects, including rocks (1974–5), caves (1975–7), women (1977) and the work of a number of other artists including Gustave Doré, Leonardo da Vinci, Franz-Xavier Messerschmidt, Vincent van Gogh, Rembrandt van Rijn and Francisco de Goya; he also used images of Greek sculptures and of mummies, death-masks and corpses to similar ends. From 1973 he collaborated on a series of works with Dieter Roth.  His return to the image of the cross in a series of monumental paintings (e.g. Cross, oil on mixed medium on cardboard, 1020×730 mm, 1980–83) attested to his interest in the relationship between life and death, between the physical and the spiritual, redemption and sacrifice. Such themes were also broached in his Hiroshima series of 72 overpainted photographs (1982), in which he drew over photographs taken after the city was destroyed by an atomic bomb in 1945. Constantly adding to his repertory of images—for example, by making use from 1985 of botanical and zoological illustrated books produced in the 18th century and 19th—Rainer continued to exploit the interaction of intellectual meditation and bodily expression.