Thomas Sieverts (born 1934) is a German architect and urban planner. He is the author of Zwischenstadt (1997; first published in English in 2000 as Cities without Cities: An interpretation of the Zwischenstadt), a book which addresses the decentralization of the compact historical European city and examines the new form of urbanity which has spread across the world describable as the urbanised landscape or the landscaped city. Sieverts calls this the Zwischenstadt, or "in-between city", as it exists between old historical city centres and open countrysides, between place as a living space and the non-places of movement, between small local economic cycles and the dependency on the world market. In 2008 a group calling itself "suddenly" commissioned the American writer Diana George to make a new translation of Zwischenstadt which they published as Where We Live Now (the English phrase George chose as the translation of Sieverts's neologism "Zwischenstadt"). In October 2008, Sieverts came to Portland, Oregon, on the occasion of the book's publication to take part in a week-long symposium about his work, also called suddenly.
Sieverts studied architecture and urban design in Stuttgart, Liverpool, and Berlin between 1955 and 1962. He became an assistant lecturer at the Technical University of Berlin. In 1965 he formed the "Freie Planungsgruppe Berlin", becoming Professor of Urban Design at the Hochschule der bildenden Künste, Berlin, between 1967 and 1970. He was briefly a guest professor in the Urban Design Program at the Graduate School of Design, Harvard University, Cambridge, Massachusetts. He was the Professor of Urban Design at the Technical University Darmstadt from 1971 to 1999, and worked also as a Professor at the School of Town Planning, University of Nottingham, from 1984 to 1989. He served as Scientific Director for the International Building Exhibition (IBA), Emscher Park, Gelsenkirchen from 1989 to 1994, and Fellow at the Institute of Advanced Studies, Berlin from 1995 to 1996.
He is currently a partner in S.K.A.T., Architekten und Stadtplaner, which began in 2000.