Pseudocapillaria nannupensis n. sp. (Nematoda: Capillariidae) is described from the intestine of the freshwater cobbler, Tandanus bostocki (Plotosidae) from the Blackwood River in the southwest of Australia. Although there have been 7 species of capillariids described from teleost fishes in Australia, this is the first species belonging to the genus Pseudocapillaria. The presence of a dorsal cuticular membrane in the male places this species in the subgenus Ichthyocapillaria. However, the dorsal cuticular membrane is only very small, so there are also some similarities with members of the subgenus Pseudocapillaria. It resembles Pseudocapillaria indica but differs by the larger size of the eggs, by the reticulated surface structure of the eggs, and by the larger spicule. It differs from Pseudocapillaria tomentosa by the structure of the proximal spicule rim and in having a vulval opening closer to the esophago-intestinal junction. It differs from Pseudocapillaria salvelini by the structure of the proximal spicule rim, in having a vulval opening closer to the esophago-intestinal junction, and by having a much shorter dorsal cuticular membrane.