Cities are the main hubs of human activity and the engines of economic growth. In pursuit of such growth, cities are transgressing their local environmental boundaries. Ongoing urbanisation increasingly contributes to the human pressure on planetary boundaries and negatively affects planetary health. In a telecoupled world, cities externalise impacts by shifting production and many other functions away from their boundaries. At the same time, urban inhabitants and people who follow urban lifestyles but live outside cities are increasingly disconnected from nature. This Viewpoint highlights the role of degrowth in keeping an urban planet within planetary boundaries and suggests areas for further research and policy. Degrowth calls for meaningfully connecting planetary boundaries with cities and ensuring everyone receives a fair share of their ecological capacity. Degrowth calls for lower use of existing resources, highlights political power asymmetries, and moves beyond pricing interventions. Degrowth addresses three key aspects that connect cities and urban lifestyles to planetary boundaries: reducing production and consumption, connecting people and nature, and including nature (to a more substantial extent) in the design of cities and in what is used and consumed in cities. A radical degrowth transformation of cities is necessary to stay within a safe operating space for humanity.