A new species of sphaeromatid isopod, Dynoides daguilarensis n.sp., is described from intertidal black mussel (Septifer virgatus) beds in the Cape d'Aguilar Marine Reserve, Hong Kong. This species has been previously misidentified as D. serratisinus and D. dentisinus. It is characterized by a pleotelsonic apex with an open, sub-parallel slit, both edges of which are serrated from the bottom to the two sides of the apex; uropod endopods which are sub-apically notched externally; and a tubercle on the ventral side of the pleonal process. The composition of Dynoides and its relationships to Clianella are reviewed. A revised composition of Dynoides with a key to the species is provided.