<p>Given the high prevalence of social media addiction among emerging adults, the current longitudinal study is the first to investigate the cross-lagged relationships among the fundamental fear (i.e., death anxiety), the specific fear (i.e., appearance-based rejection sensitivity [appearance-RS]) and social media addiction over time, including the bidirectional associations between each fear and social media addiction, and the mutual influence between the two fears. Through online surveys, a total of 801 Chinese emerging adults (559 women) aged 18–23 years (<i>M</i> = 19.02, <i>SD</i> = 0.88) completed both waves of data collection and were successfully matched across the two time points in a one-year longitudinal study. Pearson correlation analysis indicated that both baseline death anxiety and appearance-RS were positively associated with follow-up social media addiction one year later. Results of the two-wave cross-lagged panel model (CLPM) using structural equation modeling (SEM) indicated that baseline appearance-RS, but not baseline death anxiety, predicted follow-up social media addiction; baseline social media addiction did not predict either form of fear at follow-up, whereas a significant reciprocal association between death anxiety and appearance-RS emerged over time. This study provides insight into developing interventions to reduce fears by addressing appearance-RS and its association with death anxiety, and incorporating these efforts into school-based programs for social media addiction prevention and reduction in emerging adults. </p>