A new ridgewayiid copepod, Stygoridgewayia trispinosa n. g. and n. sp., is described from continental ground waters of the Cape Range Peninsula and Pilbara regions of Western Australia, as the first occurrence of this family in fresh waters. The new genus can be distinguished from other ridgewayiid genera by a combination of characters that include three digitiform processes and striated membrane on each caudal ramus, fused ancestral segments II-IV and V-VI on the antennary exopod, leg 1 with a subterminal flagellum on the outer spiniform setae of the terminal exopodal segment, legs 3 and 4 with two outer spiniform setae on the terminal exopodal segment, and female leg 5 with reduced armature on the exopod, terminal exopodal segment inserted along the distal margin of the middle exopodal segment, and endopod absent. We postulate that this new ridgewayiid is a particle feeder living in close contact with the sediment surface, and originated in shallow coastal waters and secondarily colonized the freshwater hypogean environment.