Demetri Porphyrios (Greek: Δημήτρης Πορφυρίου; born 1949) is a Greek architect and author who practices architecture in London as principal of the firm Porphyrios Associates. In addition to his architectural practice and writing, Porphyrios has held a number of teaching positions in the United States, the United Kingdom and Greece. He is currently a visiting professor at the Yale School of Architecture.

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While Porphyrios is considered to be an exponent of New Classical architecture, he has designed buildings in both the Gothic and classical idioms.

Porphyrios studied at Princeton University where he earned a M.Arch. (Master of Architecture), and a Ph.D. in the history and theory of architecture. He wrote his Ph.D. thesis on the Finnish architect Alvar Aalto, in which he described the themes he believed had generated Aalto's work (typology, urbanism and nature), while arguing that Aalto's work was the end of the line for modernist architecture. Published later as Sources of Modern Eclecticism (London: Academy Editions, 1982), the book was presented as a structuralist analysis of Aalto's architecture; its author described as having been influenced by the philosophers Michel Foucault and Louis Althusser.

In his writing, Porphyrios has advocated a "classicism without style" (which he called "Doricism"), similar to the Nordic Classicism that prevailed in early 20th-century Scandinavia in the work of architects such as Kay Fisker in Denmark, Gunnar Asplund in Sweden, and in the early work of Alvar Aalto in Finland.

In the 1980s, Porphyrios regularly contributed to the journal Architectural Design, advocating the classical and vernacular as rational architectural languages. In 2002, Princeton University commissioned him to design a residential college (Whitman College) in the Collegiate Gothic style, which was completed in 2007. Following Princeton, Porphyrios began work on Selwyn College, Cambridge and Magdalen College, Oxford, adding his neoclassical style to the two ancient British universities.

• The Pavilion, The Battery, New York, USA (1992).
• Pitiousa town, Spetses, Greece (1993)
• Three Brindleyplace Office Building, Birmingham, UK (1998)
• The Grove Buildings, Magdalene College, Oxford, UK (1999)
• Duncan Galleries, Nebraska, USA (2002)
• Interamerican Headquarters Office Building, Athens, Greece (2002)
• Ann's Court, Selwyn College, Cambridge, UK (2003)
• Seven Brindleyplace Office Building, Birmingham, UK (2004)
• Whitman College, Princeton University, USA (2007)

• Princeton Weekly Bulletin article on Whitman College
• Porphyrios Associates
• Yale School of Architecture