Social anxiety disorder (SAD) and eating disorders (EDs) are highly comorbid. Negative self-portrayal, or 'perceived flaws in the self,' is a key feature of SAD and consists of three self-critical aspects that are presumed to be flawed: social competence, physical appearance, and signs of anxiety. Negative self-portrayal has yet to be studied among EDs, despite research suggesting that individuals with EDs have a greater negative self-image and self-criticism. The identification of negative self-portrayal as a shared risk factor for SAD and EDs may have important implications for both prevention and treatment. The current study (N = 300 undergraduate women) aimed to extend negative self-portrayal theory to the EDs. We found that several aspects of negative self-portrayal were related to ED severity and that physical appearance concerns predicted subsequent ED severity. In a cross-sectional model of shared vulnerability, we found that physical appearance concerns were a shared correlate of SAD and ED symptoms. In a prospective shared vulnerability model, we found that (a) physical appearance prospectively predicted ED severity and (b) symptoms of SAD prospectively predicted all aspects of negative self-portrayal. Concerns about flaws in appearance should be targeted in the treatment of EDs, as this aspect of negative self-portrayal was shown to predict subsequent ED severity. Future research should further investigate the theory of negative self-portrayal as it pertains to EDs.