The study of personality in animals is a rapidly growing scientific field and numerous species have been reported to show consistent personality profiles. Much animal personality research has focused on nonhuman primates, with the main emphasis being placed on Old World primates, particularly rhesus macaques and chimpanzees. So far, little work has been done on cooperatively breeding nonhuman primates and New World species. Here, we study personality in the cooperatively breeding common marmosets (Callithrix jacchus) to broaden the taxonomic range of such research and to widen the perspective of comparative personality research. We use behavioral data collection and observer trait ratings to assess marmoset personality dimensions. The resulting behavioral and rating-derived personality dimensions, when viewed in tandem, resemble the human five-factor model and include extraversion, agreeableness, neuroticism, openness, and conscientiousness. Correlations between the behavioral data and the observer trait-rated personality components suggest that the personality construct of common marmosets exhibits both convergent and discriminant validity. The finding of a distinct Conscientiousness component in this species extends previous knowledge in comparative personality psychology and warrants reconsideration of proposed taxonomic trait distributions.