<p>In an increasingly urbanized world, urban biodiversity is people’s primary contact with nature. However, as cities expand and densify, urban green and blue spaces and their biodiversity are under pressure, risking declines in urban liveability. This Review discusses the benefits of urban biodiversity and the multiple challenges it faces, and identifies opportunities and pathways towards developing sustainable, biodiverse cities for both humans and nature. The substantial biological richness that urban areas can harbour helps to mitigate environmental pressures, address and adapt to climate change, and benefits human health and well-being. However, urban biodiversity is challenged by competition for space, human pressures and the declining engagement of urban residents with nature. Understanding the underlying mechanisms of both the benefits and challenges of urban biodiversity informs efforts to create and maintain high-quality urban blue–green infrastructure. Biodiversity-sensitive and socially inclusive urban governance and urban planning are key to developing biodiverse, green cities. Urban policies should move towards cross-sectional approaches that coordinate planning for biodiversity and green spaces with sectors such as health, education, urban planning and design. Developing cities as shared environments for humans and nature contributes to global biodiversity conservation and offers solutions to the social and environmental challenges increasingly faced by cities.</p>