The present study sought to examine the mediating role of social anxiety in the relationship between family functioning and loneliness among senior high school girls in Babolsar. The research was applied in nature and employed a descriptive-correlational method using structural equation modeling. The statistical population was comprised of all senior high school girls with an average age of 16 in public schools in Babolsar during the 2019-2020 academic year. A total of 252 students were selected through two-stage cluster random sampling based on Krejcie and Morgan's table. They completed the Family Assessment Device Questionnaire (Epstein et al., 1983), the Social Anxiety Scale (Connor et al., 2000), and the Loneliness Scale (Asher et al., 1984). Data analysis was conducted using structural equation modeling with SPSS26 and SmartPLS2 software. The findings indicated that the proposed model had a good fit and that family dysfunction had a direct effect on social anxiety and loneliness. Additionally, social anxiety had a direct effect on loneliness. Moreover, family functioning influenced loneliness through the mediation of social anxiety. According to the results, family dysfunction leads to increased social anxiety and loneliness. Therefore, it is essential for specialists to pay attention to predictive variables and their impact on family relationships.