A new species of pseudocrangonyctid amphipod, Pseudocrangonyx wonkimi sp. nov., was found in the groundwater of a cave in the southwestern Korean Peninsula. Pseudocrangonyx wonkimi sp. nov. is morphologically most closely related to P. joolaei Lee et al., 2020. However, P. wonkimi is clearly distinguished from P. joolaei by lacking sternal gills, fewer setae on maxilla 1 inner plate, fewer serrate robust setae on the carpus of the gnathopods, lacking bifid setae on the inner ramus of pleopod 3, and fewer articles of rami on pleopod 3. We also determined sequences of mitochondrial cytochrome c oxidase subunit I (COI) of P. wonkimi sp. nov. for molecular diagnosis. From the molecular analysis based on COI sequences, P. wonkimi showed the closest relationship with P. joolaei with 15.1% genetic distance.