The design of urban spaces, including waterfronts, must consider the flexibility to respond to urban and environmental changes and the continuity that respects the existing urban factors that have determined the essential characteristics of the place. The Hangang River in Seoul, South Korea, has been susceptible to significant changes over the years and has accumulated considerable historical traces and an irrevocable relationship with its urban contexts. On the other hand, as the country experienced rapid development, most urban plans have been executed based on shortsighted perspectives without considering sustainability. This study suggests adaptability as the main driver to achieve sustainability of the urban riverfront by viewing the site condition dichotomously. The components of the site were distinguished as fixed and variable factors. Considering fixed factors, six design intervention types were applied to the site through extensive macroanalysis and correlation studies, and each intervention was proposed to respond to the variable factors of the site. This study proposed four strategies to upgrade the riverfront as an adaptable space. First, five types of waterfronts were introduced with more distinct functions and identities to accommodate various activities. Second, commercial development was introduced to build a stable financial base that could respond to economic fluctuations. Third, the ecological waterfront space was expanded to react to climate change in the long term and achieve the goals of improving the microclimate, biodiversity, and educational function in the short term. Lastly, connective infrastructure was introduced to address the rapidly increasing demand for open space. The significance of this study was twofold. First, the site condition was interpreted more comprehensively by applying a framework for analysis by distinguishing the fixed and variable factors. Second, the strategy presented in this study can be applied to establish a waterfront redevelopment plan in other cities that have a context similar to the Hangang River of Seoul.