We report a case of jerk seesaw nystagmus that developed after posterior cranial fossa decompression with cerebellar tonsillectomy for Chiari I malformation. A 40-year-old female was diagnosed to be suffering from a Chiari I malformation associated with syringomyelia. Posterior cranial fossa decompression with cerebellar tonsillectomy was performed. Three months later, she developed oscillopsia and jerk seesaw nystagmus. The oscillopsia improved substantially with oral gabapentin. We conclude that oscillopsia and acquired nystagmus are rare but potentially disabling complications following posterior cranial fossa decompression. Oral gabapentin may be effective in the management of oscillopsia secondary to acquired jerk seesaw nystagmus.