During a study of the harpacticoid copepods from the intertidal mudflat of Ganghwa Island, in the Yellow Sea of Korea, a new species of the genus Nannopus was discovered. Nannopus ganghwaensis shared the character of seven elements in P4 exp-3 with N. flexibilis Lilljeborg, 1902, N. didelphis Fiers & Kotwicki, 2013, and N. hirsutus Fiers & Kotwicki, 2013. However, the new species was clearly distinguished from the three congeners by the combination of the following character states: (1) absence of the integumental window on the dorsal surface of cephalothorax, (2) the lateral margins of urosomites is not densely hirsute, (3) the distal small seta of P4 endopod is naked, (4) the innermost seta of P5 exopod fused to the segment, (5) the inner most distal seta of P2 enp-2 is 1.7 times longer than the outer spine of its segment, and (6) caudal seta IV slightly inflated at its insertion site, and pinnate. Additionally, the male of new species differed from N. didelphis by the following characters: (1) the P2 exp-2 with an inner seta, (2) the inner most seta of P2 enp-2 is pinnate, (3) the distal pinnate seta of P3 enp-2 is 2.6 times longer than the one in N. didelphis, (4) P6 is asymmetrical with one spermatophore at one side, and without a notch at the distal margin of P6. The male of new species also differed from N. flexibilis in having the sharper distal apophysis in P3 enp-2. A key to the species of Nannopus including new species and eight valid congeners is provided.