A 16-year-old girl with no prior medical history developed vertigo and nausea following alimentary infection. Neurological examination showed limb and truncal ataxia, opsoclonus, myoclonus, and hyperreflexia. Brain magnetic resonance imaging and cerebrospinal fluid analysis showed no abnormalities. Treatment with i.v. high-dose methylprednisolone and immunoglobulin was started, but this proved ineffective. The clinical course was unusual, so whole-body computed tomography was done to evaluate other differential diagnoses. Imaging identified right ovarian mature teratoma. Paraneoplastic opsoclonus-ataxia syndrome was suspected, therefore single-incision laparoscopic ovarian cystectomy was done 10 days after admission. Two months after therapy, the patient had complete recovery and remained asymptomatic at 1 year after onset. Serum testing for anti-neuronal antibodies was negative, including for anti-N-methyl-d-aspartate-receptor antibody. Young women with ataxia and opsoclonus of unclear etiology should be examined for the presence of ovarian teratoma, then intensive immunotherapy and prompt tumor resection can lead to good clinical outcome.