This study reports the discovery of a neck muscle: the “accessory cleidohyoid muscle,” identified during the anatomical dissection of the infrahyoid muscles in a 90-year-old male cadaver. The muscle was unilateral (on the left) and lateral to the omohyoid muscle, originating from the middle third of the clavicle and inserting into the body of the hyoid bone. This muscle, larger than the omohyoid muscle, was classified as the accessory cleidohyoid muscle, co-existing unilaterally with the superior belly of the omohyoid muscle. No symmetrical muscle was found on the right side of the neck. This case report underscores the clinical implications of this rare muscular variation that can cause diagnostic confusion and significant surgical complications. The findings highlight the need for increased awareness of this additional muscle.