The Topological City is a design concept for a three-dimensional city in China. To accommodate future populations, cities need to be dense, diverse and ecological. Minimising the physical dimension reduces the consumption of material and energy and the destruction of ecological and agricultural landscapes. Mixed-use cities reduce the time, energy and congestion of travel. The integration of natural systems improves environmental quality and supports biodiversity. Ultimately however, the quality and functionality of contemporary urban life relies on dynamics of economic, social and intellectual interaction. The design form of a city must facilitate exchange between heterogeneous and changing programs while minimising energy and maximising habitat. High-rise construction is the dominant morphology of modern cities. Generated by an extrusion from the ground toward the sky, high-rise adds significant density but results in circulatory dead ends, structural inefficiency and habitat disconnectedness. High-rise is not interactive, energy efficient, or ecological. The Topological City proposes an alternate configuration of the urban environment. Drawing on the theory of place and connection known as ‘topology’, it constructs a three-dimensional network of vertical and horizontal towers, connected and activated by circulation. The Topological City is dense, diverse, ecological and interactive, suggesting a potential new direction for sustainable urban form.