This study examined whether the therapeutic relationship in music therapy with children with Autism Spectrum Disorder predicts generalized changes in social skills. Participants (4-7 years, N = 48) were assessed at baseline, 5 and 12 months. The therapeutic relationship, as observed from session videos, and the generalized change in social skills, as judged by independent blinded assessors and parents, were evaluated using standardized tools (Assessment of the Quality of Relationship; ADOS; SRS). Linear mixed effect models showed significant interaction effects between the therapeutic relationship and several outcomes at 5 and 12 months. We found the music therapeutic relationship to be an important predictor of the development of social skills, as well as communication and language specifically.