Objective. Alexithymia, a trait dimension defined by difficulties identifying and describing feelings and an externally oriented thinking style, is often associated with negative moods such as depression, anxiety, and stress as well as an insecure adult attachment style. The present study evaluated the hypothesis that the link between alexithymia and negative moods is mediated by insecure attachment, which implies a lack of social resources for coping with distress. Method. A nonclinical online community sample of 206 male and female young adults recruited from throughout Australia completed validated measures of alexithymia, attachment security, and negative moods. Results. In a hierarchical regression model with demographic variables and socially desirable responding at step 1, alexithymia at step 2, and insecure attachment at step 3, the final model accounted for 62.4% of variance in negative moods, with alexithymia and insecure attachment as significant predictors. Bootstrapped mediation modelling, controlling for demographic variables and socially desirable responding, indicated partial mediation of the association between alexithymia and negative moods by insecure attachment. Conclusions. Results point to the role of insecure attachment in the negative moods often associated with alexithymia, presumably in addition to the deficient emotional self-regulation associated with alexithymia.