This study examines the interaction of collective personality (i.e., the Big Five traits) and national diversity on group interpersonal citizenship behavior. Groups composed of diverse nationalities are theorized to manifest fewer initial shared understandings, enabling them to obtain more benefits than homogeneous groups from collective personality traits that promote better quality relationships. In a study of university students assigned to 15 nationally homogeneous groups (n = 61) and 20 nationally heterogeneous groups (n = 79), groups scoring higher on openness to experience, agreeableness, or extraversion show more interpersonal citizenship behavior. The effects of openness to experience, agreeableness, and extraversion are enhanced for nationally diverse teams. Moreover, neuroticism had no direct effect on interpersonal citizenship behavior; its interaction with national diversity yielded reversed effects. Neuroticism was negatively related to citizenship behavior in diverse groups and positively related to it in homogeneous groups. Theoretical and practical implications are developed for small group situations where diversity issues may threaten relationship quality.