Adolf Franz Karl Viktor Maria Loos (December 10, 1870 – August 23, 1933) is an Austrian and Czech architect and influential European theorist of modern architecture.

His essay Ornament and Crime advocates smooth and clear surfaces in contrast to the lavish decorations of the fin de siècle, as well as the more modern aesthetic principles of the Vienna Secession, exemplified in his design of Looshaus, Vienna. Loos became a pioneer of modern architecture and contributed a body of theory and criticism of Modernism in architecture and design and developed the "Raumplan" (literally spatial plan) method of arranging interior spaces, exemplified in Villa Müller in Prague.

Loos has three tumultuous marriages that all end in divorce.

He suffers from poor health, including an inherited hearing affliction.

He is convicted as a pedophile in 1928 for exploiting girls from poor families, aged eight to ten.

He dies aged sixty-two on August 23, 1933, in Kalksburg near Vienna.