Abstract: Earthquake early warning systems (EEWSs) are critical for mitigating seismic hazards and reducing losses. Conventional EEWSs, which rely on dense sensor networks in fixed locations, are often cost-prohibitive and difficult to deploy, particularly in resource-constrained regions. The advent of low-cost mobile sensing technologies, particularly micro-electromechanical system sensors in smartphones, has enabled the development of crowdsourced mobile-phone-based EEWSs. These systems capitalize on the ubiquity and portability of personal devices for seismic detection and offer advantages such as cost-effectiveness, rapid deployment, and enhanced spatial coverage in infrastructure-limited seismic zones. In this review, we discuss the fundamental principles, key technologies, and global development of crowdsourced smartphone-based EEWSs (Sp-EEWSs), as well as future research directions. We also highlight the operational performance, adaptability, and advantages of Sp-EEWSs over those of traditional EEWSs. Despite their potential, Sp-EEWSs face obstacles that impede their widespread adoption, including technical risks, reliability issues, public skepticism, and implementation barriers. Addressing these issues is essential for leveraging Sp-EEWSs to bolster global earthquake preparedness and disaster response strategies. We also explore recent advancements in technological development, system integration, user engagement, management structures, and multifunctional applications to improve the accuracy and efficiency of Sp-EEWSs. Overall, this review provides a comprehensive reference for researchers and practitioners aiming to advance EEWS capabilities through innovative and scalable solutions.