This case report describes a rare side effect associated with nasal continuous positive airway pressure (nCPAP), masticatory muscle myalgia. A 69-year-old man was referred to a clinic specializing in orofacial pain and dysfunction with complaints of pain in the area of the masseter muscles. He was diagnosed with severe obstructive sleep apnea (OSA) 9 months earlier, for which he received nCPAP. Six months ago, his pain complaints started. The pain was diagnosed as masticatory muscle myalgia, due to sleep-related tooth clenching and intense tongue thrusting. These activities were likely related to the patient's attempts to prevent leakage through the oral cavity of the airflow generated by the nCPAP device. The patient did not complain about leaking of the nasal mask itself. Considerable alleviation of the pain complaints as well as reduction of the clenching and thrusting behavior were achieved by substituting the nasal mask with a full face mask. Sleep medicine physicians should be aware of this possible side effect of nCPAP.