Opisthotonus as a presenting feature in neurosurgical patients is rare, with few reports describing such presentations. Only four reports of opisthotonos secondary to posterior fossa mass were identified. An unclear pathophysiology, and broad aetiology contribute to clinical misdirection. While posterior fossa lesions commonly present with signs of raised intracranial pressure, or cerebellar dysfunction, this case describes the presentation of an infant with opisthotonic posturing, ataxia and autonomic dysfunction secondary to a large pilocytic astrocytoma. Despite initial treatment of hydrocephalus, opisthotonus only resolved with complete surgical resection of the posterior fossa mass. At follow-up, the child remains well and without signs of hypertonicity or other signs or symptoms. Presentations involving opisthotonus are rare, and active exclusion of posterior fossa pathology is necessary. In this case, urgent surgical resection allowed for a positive patient outcome. Description of such a case may contribute to understanding of similar presentations in the neurosurgical context.