The International Style is a major architectural style and movement that began in western Europe in the 1920s and dominated modern architecture until the 1970s. It is defined by strict adherence to functional and utilitarian designs and construction methods, typically expressed through minimalism. The style is characterized by modular and rectilinear forms, flat surfaces devoid of ornamentation and decoration, open and airy interiors that blend with the exterior, and the use of glass, steel, and concrete.

The International Style is sometimes called rationalist architecture and the modern movement, although the former is mostly used in English to refer specifically to either Italian rationalism or the style that developed in 1920s Europe more broadly. In continental Europe, this and related styles are variably called Functionalism, Neue Sachlichkeit ("New Objectivity"), De Stijl ("The Style"), and Rationalism, all of which are contemporaneous movements and styles that share similar principles, origins, and proponents.

Rooted in the modernism movement, the International Style is closely related to modern architecture and likewise reflects several intersecting developments in culture, politics, and technology in the early 20th century. After being brought to the United States by European architects in the 1930s, it quickly became an "unofficial" North American style, particularly after World War II. The International Style reached its height in the 1950s and 1960s, when it was widely adopted worldwide for its practicality and as a symbol of industry, progress, and modernity. The style remained the prevailing design philosophy for urban development and reconstruction into the 1970s, especially in the Western world.

The International Style was one of the first architectural movements to receive critical renown and global popularity. Regarded as the high point of modernist architecture, it is sometimes described as the "architecture of the modern movement" and credited with "single-handedly transforming the skylines of every major city in the world with its simple cubic forms". The International Style's emphasis on transcending historical and cultural influences, while favoring utility and mass-production methods, made it uniquely versatile in its application; the style was ubiquitous in a wide range of purposes, ranging from social housing and governmental buildings to corporate parks and skyscrapers.

Nevertheless, these same qualities provoked negative reactions against the style as monotonous, austere, and incongruent with existing landscapes; these critiques are conveyed through various movements such as postmodernism, new classical architecture, and deconstructivism.

Postmodern architecture was developed in the 1960s in reaction to the International Style, becoming dominant in the 1980s and 1990s.

The term "International Style" was first used in 1932 by the historian Henry-Russell Hitchcock and architect Philip Johnson to describe a movement among European architects in the 1920s that was distinguished by three key design principles: (1) "Architecture as volume – thin planes or surfaces create the building's form, as opposed to a solid mass"; (2) "Regularity in the facade, as opposed to building symmetry"; and (3) "No applied ornament".

International style is an ambiguous term; the unity and integrity of this direction is deceptive. Its formal features were revealed differently in different countries. Despite the unconditional commonality, the International Style has never been a single phenomenon. However, International Style architecture demonstrates a unity of approach and general principles: lightweight structures, skeletal frames, new materials, a modular system, an open plan, and the use of simple geometric shapes.