River herring populations, including Alosa pseudoharengus and Alosa aestivalis, have significantly declined as a result of anthropogenic factors throughout their range in eastern North America. To better understand the health of the species, parasite surveys were conducted in several New Jersey rivers. A novel myxozoan parasite, Myxobolus mauriensis n. sp., is described infecting the cartilage of pleural ribs in young-of-the-year fish. The parasite forms large polysporic plasmodia forming pseudocysts within the ribs, which extend into the musculature. Pathology associated with infection includes costochondritis, breaks in the rib bones, and deformed bone growth. Rupture of large pseudocysts and release of mature spores are associated with myositis, dermatitis, and peritonitis. Phylogenetic analysis reveals that M. mauriensis n. sp. occurs in a long-branching clade basal to other myxobolids, grouping with several Myxobolus species from marine fish (Myxobolus groenlandicus, Myxobolus aeglefini, and Myxobolus albi). The closest identity is to M. groenlandicus, with 83% identity based on a 1,762-bp sequence of the SSU 18S rDNA. Similarly, spore morphology, tropism for cartilage, and association with marine/brackish environments are shared in these 4 species. Mature spores of M. mauriensis n. sp. are similar to other reported myxobolids, though spores are slightly wider (12.1 ± 0.44 μm) than long (11.4 ± 0.44 μm), with a length:width relationship of 0.94 (±0.04), a feature not commonly described for other species of Myxobolus. Prevalence was studied by histology and fresh observation. In both Blueback Herring and Alewife, the highest infection prevalence occurred in the Maurice River at around 20%, and lower prevalence was found in the Great Egg Harbor River at around 5%. In the Delaware River, prevalence was about 2% in Blueback Herring, while the parasite was not detected in Alewife samples.