A new species of the alpheid shrimp genus Potamalpheops Powell, 1979 is described from an anchialine cave on Nggela Pile Island, Solomon Islands. Potamalpheops nazgul sp. nov. belongs to the P. monodi (Sollaud, 1932) species group, defined by the presence of two pairs of cuspidate setae on the distal margin of the telson and feebly or non-enlarged chelipeds. The new species is morphologically closer to P. pininsulae Bruce & Iliffe, 1992 and it is characterised among others by the presence of a well-developed antero-mesial tubercle on the eyestalk, a short rostrum with a dentate ventral carina, a toothed distal margin of first antennular segment and a non-strongly inflated carapace. The description of this new species raises the number of known species of Potamalpheops to fifteen.