A new species of Setostephanolaimus Tchesunov, 1994, S. longiseta sp. nov. was discovered in an intertidal sand beach along the Rizhao coast of the Yellow Sea. It is characterized by its long and slender body, long cephalic setae (longer than 20 μm and 16 μm in males and females, respectively) and subcephalic setae, long spicules (longer than 90 μm), gubernaculum with dorsal hooked apophyses, along with presence of 10–12 tubular precloacal supplements in males. Updated dichotomous key for species of the genus Setostephanolaimus is also given.