Glioblastoma is a highly aggressive brain tumour with a high risk of recurrence after surgery, even when combined with chemotherapy and radiotherapy. A major barrier to lasting treatment is the tumour's immunosuppressive environment, which is largely dominated by myeloid cells. Here we describe the development of a biodegradable implant to sustainably release immune-modulator small molecules to reprogram tumour-infiltrating myeloid cells toward a pro-inflammatory, antitumour phenotype in the surgical cavity after tumour removal. In immunocompetent mouse models, this therapy induces interleukin-12 expression in myeloid cells without systemic cytokine elevation, and increases the infiltration of CD8⁺ and CD4⁺ T cells. Over 50% of mice treated (in combination with radiotherapy and chemotherapy) remain tumour-free during the experimental course (80 days). We further treated human glioblastoma explants ex vivo with the therapy and observed increased interleukin-12 expression in tumour-infiltrating myeloid cells, supporting the translational potential of this strategy. This implantable system offers a promising approach to prevent glioblastoma recurrence by activating innate immunity and sustaining immune surveillance post-surgery.