Abstract: The cities of the Pearl River Delta (PRD) have been experiencing an unprecedented urban expansion for the past four decades, leading to emergence of one of the most populous and dynamic urban regions. However, these rapidly expanding cities located in a low-lying delta area also face increasing flood risk due to a combination of anthropogenic and natural factors. We use the concept of boundary spanning in combination with an institutionalist perspective to shed light on the barriers and opportunities for development of adaptive capacity in the face of that risk in Hong Kong and Guangzhou. As recognised in the flood risk management literature, such boundary spanning is necessary to effectively address the challenge of spatial adaptation to the growing flood risk, as it entails, for instance collaborating between policy sectors (horizontal boundaries), across levels of government (vertical boundaries) and between short-term and long-term planning agendas (temporal boundaries). Through the prism of institutions (e.g. planning system), ideas (e.g. dominant values in planning) and interests (e.g. rational choice-driven strategic behaviour of the actors involved), we assess how contextual institutional and cultural factors matter for the ability of those cities to address the growing flood risk in the face of climate change. The study builds on analysis of spatial planning and flood risk management policy documents, interviews with practitioners and experts, and site visits. Our findings show that due to institutional lock-ins and conflicting policy goals, horizontal boundary spanning remains hindered in both cases, despite emerging policy innovations, such as the Sponge City Plan in Guangzhou or the rollout of multi-functional and Nature-Based Solutions in Hong Kong. The responsibilities of institutions in both cities remain blurred, ‘planning for growth’ ignores flood and climate risk issues, and urban expansion into vulnerable areas continues. Important differences, however, exist in terms of vertical boundary spanning, pointing to different policy implications for each of the two cities. Wordcount: 3223