This study aims to investigate the mediation effect of narcissism and the mediation effect moderated by gender in the effect of social anxiety on university students' SNS addiction. In this cross-sectional survey, university students, aged 19 to 29 were selected from two provinces in South Korea. The sample size was calculated using G*power 3.1., and a sample of 170 university students was used in the final analysis. To perform the analysis, descriptive statistics; independent t-test, one-way ANOVA, and Pearson's correlation were used. The data collected was statistically analyzed using SPSS Program 23.0 and SPSS PROCESS macro (version 4.0). The moderated mediation effect was significant in both male and female groups. The mediation effect of narcissism on the relationship between social anxiety and SNS addiction proneness was stronger in the female group than in the male group. The findings have the potential to provide substantial basic data for developing health promotion and education programs to reduce university students' social anxiety, narcissism, and SNS addiction.