Introduction The spinal accessory nerve (SAN), traditionally recognized as the 11th cranial nerve, consists of a cranial root arising from the medulla oblongata and a spinal root arising from the upper cervical spinal cord. Despite more than a century of investigation, significant uncertainty persists regarding its precise cervical segmental origins, segmental contribution, and dorsal rootlet pattern, which are critical for safe surgical navigation during skull base procedures, posterior cervical exposure, and neck dissections. This study aims to document the length, segmental origins, and dorsal rootlet contributions of the spinal component of the spinal accessory nerve in adult cadavers. Methods Thirty formalin-fixed adult cadavers (60 sides) underwent standardized posterior craniovertebral dissection. The spinal portion of the spinal accessory nerve was exposed from the caudal cervical segment to the jugular foramen of the skull. Rootlets from C1 to C7 were identified, and the spinal root length was measured using a digital Vernier caliper. The segmental and dorsal spinal rootlet contributions to the formation of the spinal accessory nerve have been documented. Data were statistically analyzed using descriptive statistics, Shapiro-Wilk tests, paired t-tests, Welch's t-tests, chi-square tests, and Wilcoxon signed-rank tests. Results The spinal accessory nerve measured 56.8 ± 17.0 mm on the right and 58.5 ± 13.1 mm on the left, with no significant side differences. The spinal segments that provide rootlets for the formation of the spinal accessory nerve were found to be the C1-C7 segments. The most frequent formation patterns were C1-C4 (right) and C1-C5 (left) patterns. Across all 60 nerves, the highest total dorsal rootlet (n = 350) contribution arose from C2 (38.3%), followed by C3 (22.6%) and C1 (19.7%). The contributions of C5-C7 were minimal. The chi-square test comparing the segmental pattern distribution between the sides was not significant. Conclusion Previous literature has not reported a detailed measurement of the accessory nerve length. In this study, the spinal accessory nerve was measured as 56.8 ± 17.0 mm on the right side and 58.5 ± 13.1 mm on the left side, with no significant difference observed between the two sides. The spinal accessory nerve receives rootlets from the cervical segments C1-C7. These findings refine the anatomical understanding of spinal accessory nerve organization and underscore the necessity of incorporating such variability into surgical planning to mitigate iatrogenic injury in cervical and skull base procedures.