Proto-Cubism (also referred to as Protocubism, Early Cubism, and Pre-Cubism or Précubisme) is an intermediary transition phase in the history of art chronologically extending from 1906 to 1910. Evidence suggests that the production of proto-Cubist paintings resulted from a wide-ranging series of experiments, circumstances, influences and conditions, rather than from one isolated static event, trajectory, artist or discourse. With its roots stemming from at least the late 19th century, this period is characterized by a move towards the radical geometrization of form and a reduction or limitation of the color palette (in comparison with Fauvism). It is essentially the first experimental and exploratory phase of an art movement that would become altogether more extreme, known from the spring of 1911 as Cubism.

Proto-Cubist artworks typically depict objects in geometric schemas of cubic or conic shapes. The illusion of classical perspective is progressively stripped away from objective representation to reveal the constructive essence of the physical world (not just as seen). The term is applied not only to works of this period by Georges Braque and Pablo Picasso, but to a range of art produced in France during the early 1900s, by such artists as Juan Gris, Jean Metzinger, Albert Gleizes, Henri Le Fauconnier, Robert Delaunay, Fernand Léger, and to variants developed elsewhere in Europe. Proto-Cubist works embrace many disparate styles, and would affect diverse individuals, groups and movements, ultimately forming a fundamental stage in the history of modern art of the 20th-century.

The building blocks that led to the construction of proto-Cubist works are diverse in nature. Neither homogeneous nor isotropic, the progression of each individual artist was unique. The influences that characterize this transition period range from Post-Impressionism, to Symbolism, Les Nabis and Neo-Impressionism, the works of Paul Cézanne, Georges Seurat, Paul Gauguin to African, Egyptian, Greek, Micronesian, Native American, Iberian sculpture, and Iberian schematic art.

In anticipation of Proto-Cubism the idea of form inherent in art since the Renaissance had been questioned. The romanticist Eugène Delacroix, the realist Gustave Courbet, and practically all the Impressionists had abandoned a significant portion of Classicism in favor of immediate sensation. The dynamic expression favored by these artists presented a challenge in contrast to the static means of expression promoted by the Academia. The representation of fixed objects occupying a space, was replaced by dynamic colors and form in constant evolution. Yet other means would be necessary to jettison completely the long-standing foundation that surrounded them. While the freedom of Impressionism had certainly jeopardized its integrity, it would take another generation of artists, not just to bring the edifice down piece by piece, but to rebuild an entirely new configuration, cube by cube.

Several predominant factors mobilized the shift from a more representational art form to one that would become increasingly abstract; one of the most important would be found directly within the works of Paul Cézanne and exemplified in a widely discussed letter addressed to Émile Bernard dated 15 April 1904. Cézanne ambiguously writes: "Interpret nature in terms of the cylinder, the sphere, the cone; put everything in perspective, so that each side of an object, of a plane, recedes toward a central point."

In addition to his preoccupation for the simplification of geometric structure, Cézanne was concerned with the means of rendering the effect of volume and space. His rather classical color-modulating system consisted of changing colors from warm to cool as the object turns away from the source of light. Cézanne's departure from classicism, however, would be best summarized in the treatment and of application of the paint itself; a process in which his brushstrokes played an important role. The complexity of surface variations (or modulations) with overlapped shifting planes, seemingly arbitrary contours, contrasts and values combined to produce a strong patchwork effect. Increasingly in his later works, as Cézanne achieves a greater freedom, the patchwork becomes larger, bolder, more arbitrary, more dynamic and increasingly abstract. As the color planes acquire greater formal independence, defined objects and structures begin to lose their identity.

The art critic Louis Vauxcelles acknowledged the importance of Cézanne to the Cubists in his article titled From Cézanne to Cubism (published in Eclair, 1920). For Vauxcelles the influence had a two-fold character, both 'architectural' and 'intellectual'. He stressed the statement made by Émile Bernard that Cézanne's optics were "not in the eye, but in his brain".

With both his courage and experience to draw from, Cézanne created a hybrid art-form. He combined on the one hand the imitative and the immobile, a system left over from the Renaissance, and the mobile on the other; together to forming a hybrid. His own generation would see in his contradictory codes nothing more than impotence, unaware of his intentions. However, the next generation would see in Cézanne greatness, precisely because of this duality. Cézanne was seen simultaneously as a classicist by those who chose to see in his work the imitation of nature and perspective, and as a revolutionary by those who saw in him a revolt against imitation and classical perspective. Timid, yet clearly manifest, was the will to deconstruct. Artists at the forefront of the Parisian art scene at the outset of the 20th century would not fail to notice these tendencies inherent in the work of Cézanne, and decided to venture still further.