Nature-based Solutions (NbS) directly address three urgent and fundamental challenges: they improve the quality of life in cities, they reduce the urban ecological footprint, and they increase cities’ capacity to adapt to climate change in a cost-effective way. By accelerating the implementation of NbS, decision-makers can help their cities adapt to the changing climate while also enhancing climate resilience and providing other critical benefits, including cleaner air and water. This report investigates the potential of nature-based solutions (NbS) to help build smart, sustainable and resilient cities, and it also offers a set of guiding principles to improve territorial governance and establish multi-level governance frameworks to increase the impact and coherence of policies and private investments. It draws from more than a decade of research and experience from G20 countries and beyond. It is informed by the rich discussions held by the energy and climate working group of the G20 in March and April 2021, as well as collateral events organized on 16 and 23 April 2021 by the Italian Presidency with the support of UNEP and UNDP. A fundamental climate transition needs to take place in cities: this report may inspire further action by national governments and city authorities to enhance Nature-based Solutions in cities to reduce their vulnerability to climate change.