Grisel's syndrome (GS) is defined as atlantoaxial rotatory subluxation/fixation not associated with trauma or bone disease, usually following head and neck infection/inflammation or ear, nose, and throat (ENT) surgery. Many conditions could lead to Grisel's syndrome, of which mumps is rarely to be seen. This report discusses a case of GS in children with Type I atlantoaxial joint subluxation and previously diagnosed mumps. A 6-year-old boy who had cervical pain and torticollis for 2 weeks was admitted to our hospital. There was no trauma and he had not had ENT surgery but was diagnosed with mumps 2 weeks previously due to swelling of the left cheek and cervical lymph node. Physical examination and computed tomography confirmed a diagnosis of Grisel's syndrome with an ADI (atlanto-dens interval) of 1.6 mm. The patient then received occipito-mandibular traction for 6 days and recovered. No recurrence was observed at 1 year follow-up. Physicians should raise awareness of this rare complication of mumps to avoid life-threatening neurological impairments owing to Grisel's syndrome.