Abstract: Objective To characterize the anatomical features of a large unnamed nerve in the posterior cervical triangle and clarify its relationship with the lesser occipital nerve. Methods We dissected 31 adult formalin-fixed cadaver head and neck specimens (62 sides). The lateral cervical region, the anterior cervical region, the sternocleidomastoid region, and the occipital region were dissected to define the anatomical features of the unnamed nerve. Results This unnamed nerve was identified in the posterior cervical triangle in 96.8% of the specimens. The main trunk of the nerve had a diameter of about 3 mm with a length of around 10 cm. The nerve arose from the anterior branch of the second cervical nerve (C2, C2-3), entered the posterior cervical triangle at 1-3 cm above the accessory nerve, and continued to ascend along or in parallel with the posterior border of the sternocleidomastoid muscle. It passed between the attachments of the sternocleidomastoid and the trapezius to the occiput and divided into 3-5 branches, which innervated the skin area between the lesser and greater occipital nerves. Conclusions We identified a large unnamed nerve in the posterior cervical triangle, for which we coined the name "long occipital nerve" based on its unique anatomical features. The discovery of this nerve can be important for local surgery and for diagnosis and treatment of related diseases.