Klippel-Feil syndrome, or brevicollis, is a complex congenital disorder caused by the improper segmentation of the cervical vertebrae. The authors present the very rare case of a patient with Klippel-Feil syndrome who presented with an intradural arachnoid cyst at the craniocervical junction. They also examine possible factors contributing to this association. A 46-year-old woman presented with complaints of progressively worsening headaches and dizziness of 18 months' duration. She also demonstrated mild bilateral upper-extremity weakness. Magnetic resonance imaging revealed fused cervical vertebrae and a dorsal intradural arachnoid cyst at the craniocervical junction, extending down to the fourth cervical level. Because of worsening myelopathy and the presence of brainstem compression, the patient underwent surgical excision of the arachnoid cyst, which was approached via a midline posterior suboccipital/upper cervical route. An endoscope was introduced through a gap between the occiput and fused upper cervical vertebrae, and the arachnoid cyst was widely fenestrated. Postoperatively, the patient has remained symptom free for more than 2 years with evidence of good radiological decompression. The authors report a unique association between craniocervical arachnoid cyst and Klippel-Feil syndrome. To their knowledge, no other cases of this association have been reported in the literature. Arachnoid cysts should be part of the differential diagnosis in the presence of worsening myelopathic symptoms or pain in patients with Klippel-Feil syndrome.