Álvaro Joaquim de Melo Siza Vieira, GOSE, GCIH, is a Portuguese architect, born 25 June 1933 in Matosinhos a small coastal town by Porto. He is internationally known as Álvaro Siza (Portuguese pronunciation: [ˈaɫvɐɾu ˈsizɐ]).

He graduated in architecture in 1955, at the former School of Fine Arts from the University of Porto, the current FAUP - Faculdade de Arquitectura da Universidade do Porto. He completed his first built work (four houses in Matosinhos) even before ending his studies in 1954, the same year that he first opened his private practice in Porto. Siza Vieira taught at the school from 1966 to 1969, returning in 1976. In addition to his teaching there, he has been a visiting professor at the Graduate School of Design, Harvard University; the University of Pennsylvania; Los Andes University of Bogota; and the École Polytechnique Fédérale de Lausanne.

Along with Fernando Távora, he is one of the references of the Porto School of Architecture where both were teachers. Both architects worked together between 1955 and 1958. Another architect he has collaborated with is Eduardo Souto de Moura, e.g. on Portugal's flagship pavilions at Expo '98 in Lisbon and Expo 2000 in Hannover, as well as on the Serpentine Pavillon 2005. Siza's work is often described as "poetic modernism"; he himself has contributed to publications on Luis Barragán.