Brainstem gliomas are not common in adults, and the treatment strategies and their outcomes are limited. Immunotherapy is emerging as a promising new modality for the treatment of these gliomas. Here, we report the first case of brainstem glioma treated with a combination of radiotherapy and autologous formalin-fixed tumor vaccine (AFTV). A 32-year-old man presented with left facial numbness and right hemiparesis, and was referred to our department. MRI and open biopsy indicated brainstem glioma, and he was specifically diagnosed with isocitrate dehydrogenase 1-mutant diffuse astrocytoma of WHO grade II. He was treated with stereotactic radiotherapy followed by AFTV three months later. MRI conducted at 42 months after the combination therapy showed a 91% decrease in tumor volume, and the regression was maintained for 5 years. Thus, combination treatment with radiotherapy and immunotherapy may prove to be a promising alternative for the treatment of brainstem glioma.