<p>Cultural industry constitutes a novel economic paradigm for advancing the sustainable development of urban economies, with cultural industry agglomeration serving as a pivotal strategy to attain high-quality growth in the knowledge-based economy. Although cultural industry agglomeration (CIA) constitutes a critical means to achieve urban economic sustainability (UES), its spatial spillover effects remain under-explored in existing literature. Taking the three major urban agglomerations in China as case studies, this research analyzes the spatiotemporal evolution of CIA and UES across 56 cities from 2003 to 2019, and investigates the influence mechanisms and spatial effects of CIA on UES through the Spatial Panel Durbin Model. Empirical findings demonstrate that both CIA and UES exhibited an overall upward trajectory throughout the study period. Although the spatial expansion trajectories of CIA and UES varied across the three agglomerations, their spatial concentration levels increased significantly, underscoring the interconnectedness in their spatial evolution patterns. Quantitatively, increments in CIA exert positive direct effects on local UES while generating notable spatial spillover effects on the UES of economically proximate cities. Heterogeneity analysis reveals distinct regional disparities: the direct effect coefficients of CIA on UES are ordered as Beijing–Tianjin–Hebei (BTH) &gt; Yangtze River Delta (YRD) &gt; Pearl River Delta (PRD). Specifically, the BTH agglomeration demonstrates significant positive direct and spillover effects, the YRD shows pronounced positive direct effects, whereas the PRD exhibits negative spillover effects—findings that mirror regional discrepancies in cultural resource allocation and factor mobility. Regarding mediating mechanisms, market potential and technological innovation primarily facilitate local economic sustainability in BTH and YRD, while in PRD, these mechanisms manifest predominantly as influences on adjacent regions.</p>