Major Depressive Disorder (MDD) is a prevalent mental illness in the world. Pharmacological treatment is typically the first choice, but Humanistic Music Therapy (HMT) may offer additional benefits when added to Treatment As Usual (TAU). This single-arm pilot study aimed to assess the effectiveness of Group HMT as an intervention for adult women diagnosed with MDD, in a psychiatric institution. The study comprised 12 outpatient women with MDD who received more than one year of pharmacological treatment. A pre-test/post-test design using the Hamilton Depression (HAM-D-21), Personal and Social Performance (PSP), and Global Assessment of Functioning (GAF) scales were employed to evaluate symptom severity and patient changes. Results indicated a significant decrease in HAM-D scores from 15.7 to 6.6 (p ≤ .005), a notable improvement in personal and social functioning (PSP scale) (p ≤ .05), and an increase in global activity level (GAF scale) (p ≤ .05). In conclusion, adult women with MDD receiving TAU in a psychiatric institution experienced a significant reduction in depressive symptoms and enhancement in global activity, personal, and social functioning when HMT was added to their treatment regimen. This study is limited by being non-randomized, unblinded, and, with a small population. A randomized controlled trials are required to confirm the benefits of HMT.