Cancer cells with a stem-like phenotype are commonly described in glioblastoma, the most common primary adult brain cancer, that are thought to be highly tumorigenic. This phenotype comprimes high self renewal capacity and resistance against chemotherapy and radiation therapy, thereby promoting tumor progression and disease relapse. Here we show for the first time that calcitriol, the hormonally active form of the “sun hormone” vitamin D3, effectively suppresses stemness properties in glioblastoma stem-like cells (GSCs), supporting the hypothesis that cal-citriol sensitizes them to additional chemotherapy. Indeed, a physiologically relevant organotypic brain slice model was used to monitor tumor growth of GSCs and the effectiveness of combined treatment with temozolomide, the current standard-of-care, and calcitriol was proven. These findings indicate that further research on applying calcitriol, a well known and safe drug, as a potential adjuvant therapy for glioblastoma is both justified and necessary.