Previous research has repeatedly shown a positive association between global self-esteem and self-concept clarity. A secure attachment style is related to the broaden-and-build cycle, which is considered to influence the relationship between self-esteem and self-concept clarity. However, few studies have so far examined the moderating effect of a secure attachment style on this association. Therefore, the present study investigated whether the association between self-esteem and self-concept clarity is moderated by an individual's attachment style using large-scale cross-sectional data. A sample of 1042 university students (692 women, 350 men; Meanage = 19.70, SDage = 1.23, range: 18–24) were recruited. They completed questionnaires including self-report measures of self-concept clarity, global self-esteem, attachment style, and confounders (personality traits and demographic variables). A moderated regression analysis showed significant interactive effects between self-esteem and attachment anxiety—that is, the positive association between self-esteem and self-concept clarity was only found among those low in attachment anxiety. The present findings suggest the importance of attachment style in influencing self-development in adolescents and young adults.