<p>Urban digital twins have emerged as transformative tools for modern city management, offering unprecedented capabilities to monitor, simulate and optimize urban systems and services. However, current implementations often rely on static sensing infrastructures, limiting their ability to capture the dynamic, human-centric and socioeconomic aspects of urban environments. Mobile crowd data—real-time, high-resolution, context-rich information generated through citizen-carried mobile devices—comprise a game-changing resource for addressing these limitations. By integrating anonymized and aggregated mobile crowd data, urban digital twins can gain richer insights into urban environments and changes, citizen behavior and socioeconomic dynamics, enabling smarter, more adaptive urban planning and decision-making. Here we discuss the role of mobile crowd data in advancing urban digital twins, emphasizing the potential of such data to construct more accurate digital representations, enhance real-time responsiveness and foster mutual adaption between citizens and cities.</p>