Patient-professional communication has been suggested to be a major determinant of treatment outcomes in psychosocial care for children and adolescents. However, the mechanisms involved are largely unknown and no longitudinal studies have been performed. Our aim was, therefore, to assess over the course of 1 year, the impact of patient-centered communication on psychosocial problems of adolescents in psychosocial care, including the routes mediating this impact. We obtained data on 315 adolescents, aged 12-18 years, enrolled in child and adolescent social or mental health care. We assessed patient-centered communication by comparing the needs and experiences of adolescents with regard to three aspects of communication: affective quality, information provision, and shared decision-making. Changes in psychosocial problems comprised those reported by adolescents and their parents between baseline and 1 year thereafter. Potential mediators were treatment adherence, improvement of understanding, and improvement in self-confidence. We found a relationship between unmet needs for affective quality, information provision, and shared decision-making and less reduction of psychosocial problems. The association between the unmet need to share in decision-making and less reduction of psychosocial problems were partially mediated by less improvement in self-confidence (30 %). We found no mediators regarding affective quality and information provision. Our findings confirm that patient-centered communication is a major determinant of treatment outcomes in psychosocial care for adolescents. Professionals should be aware that tailoring their communication to individual patients’ needs is vital to the effectiveness of psychosocial care.