Amid the compounded impacts of climate change, economic volatility, and sudden shocks, small and medium-sized cities (SMSCs) have become a critical yet frequently overlooked weak link in urban resilience research. Objective assessment of resilience in SMSCs is essential for improving the design and effectiveness of resilience-building policies. Following China’s official city-size classification criteria—i.e., using urban-district resident population as the statistical basis and defining SMSCs as cities with less than 1 million urban-district residents—this study examines 510 Chinese SMSCs from 2012 to 2023. An entropy-weighted Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method is employed to construct an urban resilience index. Subsequently, spatiotemporal disparities are characterized using nonparametric kernel density estimation and Dagum Gini coefficient decomposition. The results indicate that: (1) from 2012 to 2023, the resilience index of Chinese SMSCs rose from 0.1108 to 0.1121, with an average annual growth rate of 0.1067%. Overall resilience remains low, and the increase is modest, showing a clear regional gradient of Eastern > Central > Western > Northeastern China. (2) Spatiotemporal differentiation reveals the fastest growth in the Eastern region, a similar trajectory in the Central region, slower growth in the Western region, and a decline in the Northeastern region, reinforcing a pattern of gradient divergence. Within-region disparities generally converged, whereas between-region disparities expanded markedly; nonetheless, the transvariation (overlapping) component remained the primary contributor to overall inequality. (3) The resilience of Chinese SMSCs may face a potential Matthew-effect risk driven by the interaction of scale disadvantage and regional disadvantage. These findings provide evidence to support policies aimed at enhancing resilience in SMSCs and inform the development of differentiated resilience-building strategies across regions.