Research has shown that social anxiety can trigger smartphone addiction, but relatively few studies have explored the specific mechanisms underlying the relationship between social anxiety and smartphone addiction. This study explores the specific mechanisms underlying the relationship between social anxiety and smartphone addiction by examining the mediating role of rumination and the moderating role of gender. In five universities, 10 classes of university students were randomly selected from each university. A total of 2500 university students participated in the actual assessment throughout this study and completed the Mobile Phone Addiction Index Scale, Ruminative Responses Scale, and Interaction Anxiousness Scale. The data were analysed using SPSS 27.0. The results of this study revealed that (1) rumination plays a partial mediating role in the relationship between social anxiety and smartphone addiction and that social anxiety not only has a direct effect on smartphone addiction but also indirectly affects smartphone addiction through rumination. (2) The mediating role of rumination between social anxiety and smartphone addiction is moderated by gender. Social anxiety is significantly and positively associated with smartphone addiction among university students, with rumination influencing this association. In addition, the indirect relationship between rumination and smartphone addiction was moderated by gender; however, the direct relationship between social anxiety and smartphone addiction was not affected by gender.