Research suggests that transgender young offenders are a uniquely vulnerable caseload that may benefit from speech pathology intervention to help bring their voice into alignment with their gender identity. However, no previous studies have investigated treatment efficacy in this population. This study investigated the impact of intensive voice feminisation therapy targeting fundamental frequency and oral resonance in a 17 year old transgender individual within a youth justice institution. Acoustic analysis, listener and self-ratings of vocal femininity, self-ratings of vocal satisfaction, a post-treatment structured interview, and pre- and post- treatment completion of the Transsexual Voice Questionnaire (TVQMtF) were utilised to determine treatment impact. Outcome measures indicated therapy was effective at increasing the client's vocal pitch and perceptually femininity without compromising vocal quality. However, the client was still not consistently perceived as female post-intervention and had difficulty implementing feminine speech strategies in discourse. This case study provides preliminary evidence for the effectiveness of intensive voice feminisation therapy in a youth offending population. This research also highlights the potential utility of speech pathologists working in youth justice settings, even when the timeframe for intervention is limited. Furthermore, this research paper validates the use of perceptual outcome measures in transgender voice work, by replicating previous findings in which significant correlations were found between perceptual ratings of vocal gender and client satisfaction.