Current research on preschool-age psychopathology suggests specific impairments in the two domains of social competence-prosocial behavior and social initiative-in children with externalizing and internalizing symptoms. While behavioral observation methods have been largely neglected in the past, they may extend the predominating questionnaire-based assessment as they allow for a precise and objective assessment of children's social behavior. In this study, we aimed to investigate whether prosocial behavior and social initiative measured in a limited resource task are related to externalizing and internalizing symptoms in a preschool-age community sample (N = 117, M = 4.67 years, SD = 2.75 months, females = 57). Externalizing and internalizing symptoms were rated by teachers (n = 109) and parents (n = 77) using the Strengths and Difficulties Questionnaire and by children using the Berkeley Puppet Interview (n = 97). Reduced prosocial actions were related to children's higher ratings of externalizing symptoms, while reduced social initiative actions were related to parents' higher ratings of internalizing symptoms. Prosocial behavior in the behavioral task was a marginally significant positive predictor of internalizing symptoms from children's perspective. These results highlight the value of behavioral observation measures and contribute to our understanding of interpersonal deficits already related to psychopathology at preschool age.