Software-enabled technologies and urban big data have become essential to the functioning of cities. Consequently, urban operational governance and city services are becoming highly responsive to a form of data-driven urbanism that is the key mode of production for smart cities. At the heart of data-driven urbanism is a computational understanding of city systems that reduces urban life to logic and calculative rules and procedures, which is underpinned by an instrumental rationality and realist epistemology. This rationality and epistemology are informed by and sustains urban science and urban informatics, which seek to make cities more knowable and controllable. This paper examines the forms, practices and ethics of smart cities and urban science, paying particular attention to: instrumental rationality and realist epistemology; privacy, datafication, dataveillance and geosurveillance; and data uses, such as social sorting and anticipatory governance. It argues that smart city initiatives and urban science need to be re-cast in three ways: a re-orientation in how cities are conceived; a reconfiguring of the underlying epistemology to openly recognize the contingent and relational nature of urban systems, processes and science; and the adoption of ethical principles designed to realize benefits of smart cities and urban science while reducing pernicious effects.This article is part of the themed issue 'The ethical impact of data science'.