Openness to diversity is useful to adjustment in the diversified modern society, but diversity can be perceived as a threat to one's identity and cultural values. Thus, tendency to feel secure and not easily threatened—attachment security—may be associated with openness to diversity. A path model hypothesized the associations between attachment insecurity (i.e., attachment anxiety and avoidance) and universal-diverse orientation (UDO; i.e., open attitude toward diversity) both directly and indirectly through low self-esteem and interpersonal trust. A structural equation modeling analysis using two samples (N = 338 and 350, respectively) of U.S. undergraduates revealed that attachment anxiety was associated with the UDO cognitive component Relativistic Appreciation positively but its emotional component Comfort with Differences negatively, suggesting ambivalent attitudes to diversity. Indirect effects were significant between attachment insecurity and UDO through trust, although self-esteem was associated with UDO only through its association with trust. It was suggested that mistrust in others' good intention might be why insecurely attached feel uncomfortable with the culturally different people.