This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/bync/ 3.0/) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.Although ganglion cyst is a relatively common benign mass in soft tissues, there have been very few reports of intramuscular ganglion cyst. We encountered such a case located in the antecubital fossa originating from the supinator muscle. A 61-year-old woman presented with a painless mass in the antecubital fossa. However, the patient complained of a significant sensory deficit in the radial side of the thumb, index, and middle finger. She also had extensor weakness at the metacarpophalangeal joint. The mass was excised completely, with no post-surgical complications. Pathologic results revealed the ganglion cyst. The patient showed improvements in sensory dysfunction and extensor weakness. Intramuscular ganglion cyst can be misdiagnosed. This report might be useful for making an accurate diagnosis and rapidly initiating treatment for an intramuscular mass.