Sándor Bortnyik (Târgu Mureș, 1893 – Budapest, 1976) was a Hungarian painter and graphic designer.

He studied at the Budapest Free School under Kernstock and József Rippl-Rónai until 1910. In 1916, he met Lajos Kassák and worked with his journal MA. He worked as its co-editor and co-curated its joint exhibition, also publishing his lino-engravings in it in 1918. 

Following the collapse of the Hungarian Soviet Republic, he fled to Vienna, and later, in 1922, he moved to Weimar upon the invite of Farkas Molnár and lived there until 1924. With Kassák's help, Bortnyik published Képarchitektúra (1921), an album of pictorial architecture, an expression of the Hungarian version of Constructivism. In 1922, he established his advertising studio Neue Reklame Gestaltung, and during the 1920s he created cutting-edge posters for advertisements.

Bortnyik never enrolled Bauhaus, although he was very active and interested in the Bauhaus and Oskar Schlemmer's theatre workshop, and attended Van Doesburg's De Stijl classes. 

This connection gave rise to Bortnyik's non-figurative works with a strong architectural component. Vivid colours and geometric form characterise his works. Bortnyik's artistic style was built on the principles of Expressionism, Cubism, Constructivism and Futurism.

Bortnyik exhibited at the Galerie Der Sturm in 1922 in Berlin and participated in the Congress of Dadaists and Constructivists in Weimar, followed by his solo show at Galerie Nierendorf in Berlin in 1923. 

Following his 1924 return to Budapest, in 1925 founded the avant-garde theatre, Zöld Szamár with Iván Hevesy and Farkas Molnár. Bortnyik founded and directed the school for advertising design Műhely from 1928 to 1938. Victor Vasarely was among his pupils. In 1927, he was the editor and designer for Uj Föld magazine, and in 1933 edited the Plakát magazine. From 1947 to 1949 he was the editor-in-chief of Szabad Művészet. He taught at the College of applied arts in 1948-49 and was the director at the College of fine arts in Budapest between 1949 and 1956.