The genus Notomastus is frequently encountered in Chinese waters. However, its species richness is poorly understood. In this study, a Notomastus species obtained from Xiamen Bay, southern China, was described and illustrated as a new species ( N. sunae sp. nov.), based on morphological and molecular analyses. The new species is characterized by having uniramous chaetiger 1, the presence of palpode and eyespots on prostomium, chaetiger 11 with notopodial capillaries and neuropodial hooded hooks, and notopodial lobes with simple epithelial extensions on far posterior abdomen. With additional specimens collected from several localities along the southern coasts of China, the morphology and geographical distribution of the new species are discussed. A key is also provided for Notomastus species with neuropodial hooks in thoracic chaetiger 11.