Background: Emotional and behavioural problems as well as a lack of social competence are common in adolescents with borderline intellectual functioning and impair their social and vocational integration. Group interventions specifically developed for this target group are scarce and controlled evaluation studies are absent. Methods: A cognitive-behavioural group training (Social Competence Training for Adolescents with Borderline Intelligence) was developed and its effectiveness was examined in students attending special vocational schools. A total of 77 adolescents with borderline intelligence were randomised either to the intervention (n = 40) or to the control group (n = 37). Outcome measures at post-treatment and at a 6-month follow-up comprised self-reports, caregiver reports and behavioural observations. Results: The adolescents in the intervention group showed temporally stable improvement in their social competence, especially in social problem solving (F = 17.6, P less than 0.001) and attainment of individual behavioural goals in everyday life (self-reports: F = 15.9, P less than 0.001; caregiver reports: F = 87.9, P less than 0.001). Effects of the intervention on other outcomes such as problem-solving competence and skills performed in standardised role plays were weak or absent. Conclusions: Social Competence Training for Adolescents with Borderline Intelligence is a promising treatment for adolescents with borderline intellectual functioning and associated emotional/behavioural symptoms. Further studies examining long-term effects on the participants' social and vocational integration are needed.