The purpose of this study was to examine whether justifying one's own social knowledge (moral, societal, psychological) toward complex social-moral issues through collaborative argumentation was associated with the improvement of social perspective taking for elementary students. A total of 129 5th graders (52% female, Mage = 10.98) from six classrooms in two public schools participated in six weekly collaborative small-group discussions to reason about complex social-moral issues such as social exclusion. Two aspects of knowledge justification were examined: the frequency of knowledge justification and the diversity in perspectives. A Poisson regression with Generalized estimation equation (GEE) revealed that frequency of knowledge justification and diversity in perspectives during collaborative argumentation were associated with pre-post changes in students' social perspective taking, as reflected in individual essays. Findings underscore knowledge justification as a potential mechanism of collaborative argumentation to promote elementary students' social perspective taking.