Marcel Lajos Breuer (/ˈbrɔɪər/ BROY-ur; May 21, 1902 – July 1, 1981) was a Hungarian-American modernist architect and furniture designer. He moved to the United States in 1937 and became a naturalized American citizen in 1944.

At the Bauhaus he designed the Wassily Chair and the Cesca Chair, which The New York Times have called some of the most important chairs of the 20th century. Breuer extended the sculpture vocabulary he had developed in the carpentry shop at the Bauhaus into a personal architecture that made him one of the world's most popular architects at the peak of 20th-century design. His work includes art museums, libraries, college buildings, office buildings, and residences. Many are in a Brutalist architecture style, including the former IBM Research and Development facility which was the birthplace of the first personal computer. He is regarded as one of the great innovators of modern furniture design and one of the most-influential exponents of the International Style.

Commonly known to his friends and associates as Lajkó (/ˈlaɪkoʊ/ LY-koh; the diminutive of his middle name), Breuer was born in Pécs, Hungary(at the time part of Austria-Hungery), to a Jewish family. He was forced to renounce his faith in order to marry Marta Erps due to anti-Semitism in Germany at the time.

Marcel Breuer left his workplace at the age of 18 in search of artistic training and, after a short period spent at the Academy of Fine Arts in Vienna, became one of the first and youngest students at the Bauhaus – a radical arts and crafts school that Walter Gropius had founded in Weimar just after the First World War. He was recognized by Gropius as a significant talent and was quickly put at the head of the Bauhaus carpentry shop. Gropius was to remain a lifelong mentor for a man who was 19 years his junior.

After the school moved from Weimar to Dessau in 1925, Breuer returned from a brief sojourn in Paris to join older faculty members such as Josef Albers, Wassily Kandinsky, and Paul Klee as a Master, eventually teaching in its newly established department of architecture.

Recognized for his invention of bicycle-handlebar-inspired tubular steel furniture, Breuer lived off his design fees at a time in the late 1920s and early 1930s when the architectural commissions he was looking for were few and far between. The structural characteristics of his wooden furniture showed the influence of Dutch designers Gerrit Rietveld and Theo van Doesburg. He was known to such giants as Le Corbusier and Mies van der Rohe, whose architectural vocabulary he was later to adapt as part of his own, but hardly considered an equal by them who were his senior by 15 and 16 years. Despite the widespread popular belief that one of the most famous of Breuer's tubular steel chairs, the Wassily Chair was designed for Breuer's friend Wassily Kandinsky, it was not; Kandinsky admired Breuer's finished chair design, and only then did Breuer make an additional copy for Kandinsky's use in his home. When the chair was re-released in the 1960s, it was named "Wassily" by its Italian manufacturer, who had learned that Kandinsky had been the recipient of one of the earliest post-prototype units.

It was Gropius who assigned Breuer interiors at the 1927 Weissenhof Estate. In 1928 he opened a practice in Berlin, devoted himself to interior design and furniture design and in 1932 he built his first house, the Harnischmacher in Wiesbaden. The house was white, with two floors and a flat roof; part of it and the terraces rose freely on supports.

In 1935, at Gropius's suggestion, Breuer relocated to London.