The current study examined the development of early adolescents' social perspective taking (SPT) through collaborative small-group discussions. A total of 250 fifth-graders were assigned to three conditions, Collaborative Social Reasoning (CSR) discussions, Read-Aloud (RA) and Regular Instruction (RI). SPT was assessed before and after the intervention using an action-based argumentation essay task through a theory-driven coding scheme aiming at capturing individuals' agentic role in SPT and SPT justification strategies. Poisson regressions with Generalized Estimating Equations based on the young adolescents' essays revealed that CSR peer group members considered more justified perspectives of the issues and provided more justification based on story events and emotional concerns for the perspectives in comparison with fifth graders in the RA and RI groups. Individuals in the RA group generated more emotional justification for their perspectives than RI members. The findings provide insights into the social process of developing SPT under collaborative small-group discussions.