Having a good relationship with one's peers is important for adolescents because it influences their self-perception. To establish a positive relationship with peers, good social skills are essential. Therefore, adolescents with social skill deficits require intervention. This study uses pre- and post-test design to examine the effectiveness of Social Skills Training (SST) implementation to improve social skills in adolescents who have peer relationship problems at school. The participant in this study was a 14-year-old girl with social skill deficits. SST was conducted in eight sessions lasting 30-60 minutes each. In this study, teachers and friends were included to assess and evaluate the participant's social skills. The subject's increased social behavior as a component of social skills was assessed by using teachers' observation and interview. The results indicate an increase in social skills in terms of the target behavior. Based on the Child's Behavior Checklist, scores in the social competence subscale increased. Meanwhile, based on the Social Skill Questionnaire, the total social skill score, along with social behavior, social perception, self-regulation, and social problem-solving scores also increased. Her peers and teachers also reported improvements in the subject's emotion regulation and social behavior. Furthermore, based on the Self-Perception Profile for Adolescent, social competence, physical appearance, close friendship, and the global self-esteem subdomains also showed an increase.