In this report, the authors present the first case of adult brainstem pilocytic astrocytoma (PA) with the H3 K27M mutation. A 53-year-old man was incidentally found to have a 2.5-cm partially enhanced tumor in the tectum on MRI. The enhancement in the lesion increased over 3 years, and gross-total removal was performed via the occipital transtentorial approach. The resected tissue indicated PA, WHO Grade I, and genetic analysis revealed the H3 K27M mutation. However, although the radiological, surgical, and pathological findings all corresponded to PA, this entity can easily be misdiagnosed as diffuse midline glioma with the H3 K27M mutation, which is classified as a WHO Grade IV tumor according to the updated classification. This case highlights the phenotypic spectrum of PA, as well as the biology of the H3 K27M–mutated gliomas, and may prove to be an exception to the rule that diffuse midline gliomas with the H3 K27M mutation behave in an aggressive manner. Based on the findings of this case, the authors conclude that, in addition to detecting the existence of the H3 K27M mutation, an integrated approach in which a combination of clinical, pathological, and genetic information is used should be applied for accurate diagnosis and determination of the appropriate treatment for diffuse midline gliomas.