Introduction: The aim of the present study was to investigate the effectiveness of Montessori-based constructivist instruction, grounded in the Fundamental Transformation Document, on the social development of elementary school students in Karaj.

Research Methodology: This study employed a quasi-experimental design with a pretest–posttest control group. The statistical population consisted of all fifth-grade female students in District 3 of Karaj during the 2021–2022 academic year. From the available schools, Hijab Girls’ Elementary School was selected as a convenient sample, and two fifth-grade classes were chosen. One class was assigned to the control group and the other to the experimental group. Data were collected using the Weitzman Social Development Questionnaire (1990). Both groups completed the pretest under identical conditions in the presence of the researcher. The educational intervention was implemented for the experimental group over 10 sessions of 45 minutes each. After the intervention, the posttest was administered to both groups under the same conditions within the specified time frame. The collected data were first analyzed descriptively to examine the status and characteristics of the sample population. Then, in the inferential statistics section, analysis of covariance (ANCOVA) was used to test the significance of the predictor variable in relation to the criterion variable.

Findings: The results showed that the posttest scores of the experimental group differed from those of the control group in social development skills, indicating a higher level of social development in the experimental group. Therefore, the reported values obtained from the multivariate analysis of covariance were presented for the social development variable.

Conclusion: Based on the results, it can be observed that the mean social development score, after controlling for the pretest score, differed between the two groups in the posttest. In addition, the values reported by the multivariate analysis of covariance for the social development variable indicate that the mean social development score, after controlling for the pretest score, showed a statistically significant difference between the two groups in the posttest.