Social sustainability, encompassing equitable development, livable urban construction, and inclusive social governance, has become a core dimension of sustainable development goals (SDGs) and urban planning practice in the context of rapid urbanization. Taking the Yangtze River Delta (YRD) urban agglomeration—the most economically developed and urbanized region in China—as the study area, this research constructed a comprehensive evaluation index system of social sustainability from three dimensions: equitable resource allocation, livable urban environment, and inclusive social development. Based on multi-source data from 2010 to 2020 (including socioeconomic statistics, remote sensing imagery, and open geospatial data), the entropy weight-TOPSIS model was used to measure the spatiotemporal evolution characteristics of social sustainability in the YRD. Additionally, the GeoDetector model was employed to identify the key driving factors and their interactive effects on the spatial differentiation of social sustainability, and the future development trends of social sustainability under three scenarios (urban expansion, ecological priority, and coordinated development) were predicted using the PLUS model. The results showed that the overall social sustainability of the YRD presented a steady upward trend from 2010 to 2020, with a spatial pattern of "high in the core, low in the periphery" and significant inter-city disparities. Equitable resource allocation was the primary constraint on social sustainability in peripheral cities, while livable urban environment was the main advantage of core cities such as Shanghai, Nanjing, and Hangzhou. The driving factor detection indicated that per capita GDP, urban green space rate, and the number of medical and educational institutions per 10,000 people were the top three key factors affecting social sustainability, with the interactive effect of any two factors showing a dual-factor enhancement pattern. Under the coordinated development scenario, the social sustainability of the YRD will achieve the most balanced and high-quality growth by 2030, with the peripheral cities narrowing the development gap with the core cities significantly. These findings imply that future urban development in the YRD should adhere to the concept of coordinated and inclusive development, optimize the spatial allocation of public resources, and promote the integrated construction of livable cities, so as to realize the high-quality social sustainability of the urban agglomeration. This study provides a quantitative method and empirical reference for the evaluation and optimization of social sustainability in China’s urban agglomerations and even in other rapidly urbanizing regions worldwide.