The present study was conducted with the aim of designing an emotional intelligence training program and investigating its effectiveness on the social adjustment of aggressive first-year high school students. Aggression is one of the common behavioral problems in the first year of high school that can have numerous negative consequences on interpersonal relationships, social interactions, social adjustment, and academic performance of students. The research method was experimental and of the pre-test-post-test design with a control group. The statistical population of the study included aggressive first-year high school students, of whom 30 were selected using a stratified random sampling method and randomly assigned to two experimental groups (14 people) and a control group (16 people). In order to measure the social adjustment of the students, the California Personality Inventory (CPI), social adjustment subscale, was used. In addition, the designed emotional intelligence training program was implemented in the form of 7 training sessions for the experimental group, while the control group did not receive any educational intervention during this period. After the intervention, the data were analyzed using descriptive statistics and analysis of covariance (ANCOVA). The results of the analysis of covariance showed that by controlling for the effect of pre-test scores, there was a significant difference between the experimental and control groups in terms of social adjustment in the post-test phase; so that emotional intelligence training led to a significant increase in the social adjustment of aggressive students in the experimental group. The research findings show that emotional intelligence training can be considered by counselors and educational planners as an effective educational intervention in promoting social adjustment and reducing aggressive behaviors in first-year high school students.