Located at the interface between cities and the sea, urban seascapes are becoming the new frontiers as sea cities adapt to sea level rise. Urban seascapes can evolve in two directions: seaward and landward. On the one hand, urban processes are increasingly expanding and intensifying in a seaward direction, driven by coastal population growth and a developing marine economy. On the other hand, the rising sea can expand seascapes landward and drive water-adaptive urban transformation. The challenges of rising sea levels and the ecological impacts of marine urbanisation, particularly from land reclamation and human-made structures at sea, create opportunities for water-adaptive and ecosystem-based solutions, among which floating solutions have emerged as a more sustainable alternative for urban expansion into the marine environment and urban adaptation in the face of flooding and landward sea rise. The recent development of floating solutions includes floating houses in the Netherlands; a prototype for an ecologically productive floating structure in San Francisco Bay; a small floating city development project along the coast of Busan, South Korea; and the Space@Sea research project conducted by 17 European partners for large-scale floating islands. The range of developments suggests that floating developments are gaining attention and significance globally. This suggests a growing need to address floating developments in spatial planning. Floating developments and, ultimately, floating cities represent an emerging form of aquatic urbanism that will require holistic spatial planning for urban seascapes and innovative planning approaches and tools to shift from land-based to water-based, from fixity-based to fluidity-based and from human demand-based to ecosystem-based systems. Several researchers have called for urban seascapes to be integrated into city planning; however, this is often constrained by land-based jurisdictions and land-based thinking. Urban planning often does not go beyond the edge of the land as city jurisdictions are limited by terrestrial boundaries; however, marine spatial planning for large ecosystem management is often developed and implemented at regional scales that pay insufficient attention to the details of urban seascapes at a local scale. The spatial planning gap in urban seascapes emphasises the need for more systematic research on how to plan urban seascapes at the frontiers of future aquatic urbanism which responds to rising sea levels. This research aims to find innovative approaches and tools for the adaptive planning of urban seascapes that supports the sustainable development of aquatic urbanism and merging of human-made and natural ecosystems. To answer the main research question ‘How can adaptive planning tools be created to help plan and develop urban seascapes more effectively for water-adaptive sea cities?’, this research explores and draws lessons from recent and current spatial planning approaches and frameworks for urban seascapes, from terrestrial urban planning and marine spatial planning for adaptability in the face of uncertainty and dynamism, and from innovative tools enabled by cutting-edge digital technologies. It does so by using two main research methods: integrative reviews of relevant scholarly and policy literature and comparative case studies from various contexts and fields. These lessons provide theoretical and practical foundations to create a novel adaptive tool for planning dynamic urban seascapes. Harnessing the power of big data and artificial intelligence, this tool will serve a wide range of stakeholders in urban seascapes, including marine authorities, planners, marine users and, in particular, users of floating structures, developers of floating developments and communities in transformation processes of sea cities. The key findings in this research make an original and significant contribution to research in this new field and offer applications for planning urban seascapes dynamically and adaptively based on the fusion of planning and developing technologies in monitoring and analytics.