Despite the exploration of detrimental consequences of parental phubbing and adolescent social networking site (SNS) addiction, research regarding their bidirectional relationships and underlying mechanisms is lacking. To address this research gap, the present study explored the longitudinal and bidirectional relationships between parental phubbing, fear of missing out (FoMO), and adolescent SNS addiction. Furthermore, the study investigated the mediating role of FoMO and the moderating role of gender therein. Data were collected among 1447 Chinese adolescents in June 2019 (T1) and January 2020 (T2). The results showed significant positive correlations among all three variables in both waves. There were bidirectional associations between parental phubbing and adolescent SNS addiction. T1 parental phubbing positively predicted T2 FoMO, and T1 FoMO positively predicted T2 SNS addiction, but not vice versa. FoMO mediated the link between parental phubbing and subsequent adolescent SNS addiction. T1 SNS addiction predicted T2 parental phubbing, and T1 FoMO predicted T2 SNS addiction – but these effects were significant only among girls. The effect of T1 FoMO on T2 FoMO was significantly stronger among girls than in boys. The findings contribute to the literature on parental phubbing and adolescent SNS addiction and provide insights for prevention and intervention programs addressing adolescent SNS addiction.