(December 10, 1870 in Brno, Moravia–August 8, 1933 in Vienna, Austria) was an early-20th century Viennese modernist architect who is associated with the International Style.

In addition to his built projects, Loos is noted for his essay/manifesto Ornament and Crime written in 1908. In it, he expressed the idea that the progress of culture is associated with the deletion of ornament from everyday objects, and that it was a crime to force craftsmen or builders to waste their time on ornamentation that served to hasten the time when an object was obsolete. Accordingly, the most primitive societies use a lot of decoration and the most advanced societies have no superfluous ornament, or at least there is benefit in suppressing ornamentation which serves no useful purpose. In the same essay Loos asserted that "All art is erotic," and that a European man who tattoos himself is either a criminal or a degenerate; if a tattooed man dies out of prison, Loos reasoned, it is only because he did not live long enough to commit his inevitable murder.

This essay is a repudiation of the work of the Vienna Secession, the Austrian version of Art Nouveau. Loos' provocative catch phrase was taken up by the Modern Movement in architecture, the other famous catch phrase of which is "form follows function". In the years between 1893 and 1896, Loos lived and worked in United States.

Loos was also interested in the decorative arts, collecting sterling silver and high quality leather goods, which he noted for their plain yet luxurious appeal. He also enjoyed fashion and men's clothing, designing the famed Knize of Vienna, a haberdashery.

Loos is also known for his notorious entry to the 1922 Chicago Tribune competition, which took the form of a single colossal Doric column.