Several studies have highlighted a relationship between attachment and theory of mind (ToM) in childhood and in clinical populations. However, little is known about the link between attachment and ToM in the general adult population. The aim of this study was, therefore, to explore how differences in attachment styles influence ToM skills in a nonclinical population of young adults. 69 young adults performed the Reading the Mind in the Eyes test, Movie for the Assessment of Social Cognition test (MASC), and an adult attachment Q-sort questionnaire. Findings revealed that dimensions of avoidant and ambivalent attachment were, respectively, related to undermentalizing and overmentalizing tendencies. Insecure participants performed significantly more poorly than secure ones on the MASC. More specifically, participants with an avoidant attachment style made significantly more responses classified as undermentalizing than those with a secure attachment style. A statistical trend was observed for the MASC no-ToM score (answer with complete lack of ToM or literal understanding), with lower scores for the insecure ambivalent group than for the secure group. These results suggest that the individuals with avoidant insecure attachment are more likely to under-attribute mental states to others, while those with ambivalent insecure attachment tend not to make attributions when the situation requires it. Implications for future research are discussed.