An 80-year-old man was diagnosed with prostate cancer in April 2014 and underwent anticancer treatment. His serum prostate-specific antigen (PSA) level was abruptly increased on December 26, 2014. He was admitted to the neurological department of our hospital on January 14, 2015, because of the appearance of staggering gait and diplopia. Neurological examination revealed marked opsoclonus, limb ataxia and ataxic gait. The patient was diagnosed with paraneoplastic opsoclonus and ataxia caused by prostate cancer relapse. Steroid pulse therapy was initiated and his symptoms, including opsoclonus and ataxia, markedly improved. Although most cases of paraneoplastic opsoclonus precede the discovery of cancer, our case developed symptoms simultaneously with relapse and acute progression of prostate cancer. Paraneoplastic opsoclonus with prostate cancer is rare. Additionally, our case showed excellent response of opsoclonus to steroid therapy without treatment of the underlying disease. (Received June 1, 2020; Accepted September 18, 2020; Published February 1, 2021).