The purpose of this paper is to investigate the links between smart cities and urban energy sustainability. Because achieving a “smart city” is a wide agenda rather than a specific set of interventions, smartness itself cannot easily be measured or quantifiably assessed. Instead, we understand smart cities to be a broad framework of strategies pursued by urban actors, and ask whether and how smart city projects catalyze urban energy sustainability. We use case studies of three cities (Nottingham, Stavanger, and Stockholm) funded by the Horizon 2020 Smart Cities and Communities program and examine how urban energy sustainability was advanced and realized through the smart city initiatives. We find first that while sustainability is not always a major objective of local implementation of smart city projects, the smartness agenda nevertheless increases the ambition to achieve energy sustainability targets. Second, the sustainability measures in smart cities are rarely driven by advanced technology, even though the smart city agenda is framed around such innovations. Third, there is significant sustainability potential in cross-sectoral integration, but there are unresolved challenges of accountability for and measurability of these gains.