<p>Previous studies have shown that there may be a strong association between basic psychological needs satisfaction (BPNS), fear of missing out (FoMO), and smartphone addiction (SA) among undergraduate students, gender differences may exist in the relationship between BPNS, FoMO, and undergraduate students' SA, and perceived social support (PSS) may moderate the relationship between BPNS, FoMO, and undergraduate students' SA. However, few studies have investigated these problems. This study used a two wave longitudinal design (with a six-month time interval) to test a moderated mediation model in which BPNS at time 1 (T1) predicted SA at time 2 (T2), with FoMO (T1) as mediator and gender and PSS (T1) as moderators. A total of 1260 undergraduate students (male: 602; female: 658) in 40 classes from five Chinese undergraduate universities were taken as subjects through cluster random sampling. The results showed that: (1) There was a significant correlation between BPNS (T1), FoMO (T1), and SA (T2). (2) FoMO (T1) partially mediated the relationship between BPNS (T1) and SA (T2). (3) Gender moderated the relationship between BPNS (T1), FoMO (T1), and SA (T2). (4) PSS (T1) could positively moderate the mediating effect of FoMO (T1) between BPNS (T1) and SA (T2). In conclusion, this study can further deepen the research on the formation mechanism of undergraduate students' smartphone addiction and provide methodological guidance for colleges and universities and parents to prevent or intervene in Chinese undergraduate students' smartphone addiction.</p>