Personality has been studied in all of the great apes, many Old World monkey species, but only a handful of New World monkey species. Because understanding the personalities of New World monkeys is crucial to understanding personality evolution in primates, we used the Hominoid Personality Questionnaire to assess personality in 55 common squirrel monkeys (Saimiri sciureus) and 40 Bolivian squirrel monkeys (Saimiri boliviensis). We found 4 personality components in each species, and labeled them Openness, Neuroticism, Assertiveness, and Agreeableness. We then, in a genus-level analysis, found 5 components, which we labeled Neuroticism, Openness, Assertiveness, Agreeableness, and Decisiveness. Comparisons of the genus- and species-level structures revealed that common squirrel monkeys had a personality structure that more closely resembled the genus-level structure than did Bolivian squirrel monkeys. We then compared the personality structures of common and Bolivian squirrel monkeys with that of brown capuchin monkeys, Sapajus apella. The personality structure of Bolivian squirrel monkeys more closely resembled that of brown capuchins. These findings suggest that the Bolivian squirrel monkey personality structure is ancestral and that Assertiveness and Openness are ancestral to both the Saimiri genus and brown capuchins; Agreeableness and Neuroticism seem to be derived in Saimiri. We discuss these findings in relation to differences in the social structures and ecologies of these species.