The lymph nodes between the sternocleidomastoid and sternohyoid muscle (LNSS) are not explicitly mentioned in the 2015 American Thyroid Association and 2008 American Head and Neck Society (AHNS) guidelines, but they are easily overlooked in papillary thyroid carcinoma (PTC). We prospectively evaluated the clinical significance of the LNSS in papillary thyroid carcinoma (PTC) patients. In five medical centers, two hundred and thirty-four PTC patients with lateral neck metastasis who underwent 264 neck dissection were enrolled in this study. LNSS was resected and used as a specimen to investigate the relationship of LNSS with several clinicopathological parameters. Of the 264 lateral neck dissections, the average lymph node metastasis rate of LNSS was 23.48%, significantly second only to that in level III (p<0.05). Univariate and multivariate analyses showed that a patient age over 45 years (OR 2.155, 95% CI 1.191 to 3.898, p = 0.011), with a tumor located in the inferior lobe of the thyroid (OR 1.517, 95% CI 1.113 to 2.068, p = 0.008), and LN metastasis at levels IIb (OR 2.298, 95% CI 1.121 to 4.712, p = 0.020) and level III (OR 2.408, 95% CI 1.222 to 4.745, p = 0.011) were independent risk factors for LNSS lymphatic metastasis. The LNSS has a high metastatic rate and is easily overlooked. Additional attention should be paid to LNSS, especially in patients over 45 years old and with PTC located in the thyroid's inferior lobe. Copyright © 2021. Published by Elsevier Taiwan LLC.