A new genus and species of symbiotic, semi-parasitic nemertean worm, Ovicides julieae n. gen., n. sp., is described from the xanthid crabs Chlorodiella nigra and C. xishaensis. The new genus possesses accessory stylet pouches and exhibits simultaneous hermaphroditism, features new to the family Carcinonemertidae. The new species has a relatively small anterior proboscis chamber and small basis, yet it possesses the largest stylet to basis ratio (up to 0.650) reported for the family. It also has a flame cell pattern of at least 2[(3 3)], which is different from those reported for two other species. The worm occurred in a lightly granulated sheath on the abdominal sterna, pleopods, and clutches of its host, but few adult worms were found. The species exhibited a high host specificity and was more frequently found on C. nigra than on C. xishaensis (7.5 vs. 0.4%, overall prevalence, respectively). Worms were only found on female crabs, and worm intensity showed a modest but significantly positive correlation with host size. Ovicides julieae represents the second genus in the Carcinonemertidae, the fifth described carcinonemertid species from Australia, and the eleventh described species in the family.