A new free-living marine nematode species of the genus Rhinema belonging to the family Monoposthiidae was collected from marine sediments in the Qiongzhou Strait of the South China Sea, and it is described here as Rhinema longispicula sp. nov. The new species is characterized by coarsely annulated cuticle with 12 longitudinal ridges (alae) marked as reversed V-shaped. The first and second annuli broader and form a kind of a cephalic capsule. The amphideal fovea circular, situated in the middle of the broad second annulus, not surrounded by dense annulations. Buccal cavity cylindrical with a large dorsal tooth. Pharynx with a prominent anterior pharyngeal bulb and an elongated terminal bulb. Elongate spicules strongly curved, with cephalated proximal end and pointed distal end. Gubaculum boat-shaped, without apophysis. A long and blunt precloacal seta present. Female with two opposed, outstretched ovaries.