A new species of rhizocephalan barnacle is described parasitizing the Florida lobsterette Nephropsis aculeata Smith, 1881. This is the first record of a nephropoid as a host for any rhizocephalan and represents the first record of a rhizocephalan parasitizing any marine lobster (Achelata, Astacidea, Glypheidea, Polychelida). The new species belongs to the genus ParthenopeaKossmann, 1874, which now contains three species: Parthenopea australisLützen, Glenner & Lörz, 2009 (on a callianassid in New Zealand), P. subterraneaKossmann, 1874 (on a callianassid and an axiid in the Skagerrak strait, Black Sea, and Mediterranean), and P. reinhardi n. sp. (on N. aculeata from Dry Tortugas, Florida, USA). The new species can be distinguished from the other two in the genus based on the shape of the seminal receptacle ducts (proximally straight in P. subterranea versus coiled in P. reinhardi n. sp.) and annuli on the stalk (present in in P. australis but absent in P. reinhardi n. sp.) and the position of the mantle opening (on the surface and close to the stalk in P. australis versus in a groove and more distant from the stalk in P. reinhardi n. sp.). We urge researchers and fisheries scientists working on marine lobsters to examine specimens for rhizocephalans, potentially leading to additional new records among lobster hosts.