Saccocoelioides beauforti (Hunter and Thomas, 1961) Overstreet, 1971, an intestinal parasite of flathead grey mullet, Mugil cephalus Linnaeus, was originally described from off North Carolina (northwestern Atlantic Ocean) and purportedly ranges south and west to Louisiana (north-central Gulf of America). We surveyed the parasites of hundreds of flathead grey mullet captured from coastal habitats of Georgia, Alabama, and Mississippi over a 25 yr period, from 1998 through 2023. Examination of whole-mounted specimens of "S. beaufort" revealed substantial morphological variation between specimens from off North Carolina to Georgia (Northwestern Atlantic Ocean) with those from off Mississippi and Alabama (north-central Gulf of America), with some overlap in Georgia. We therefore investigated the identities of these 2 groups of parasites that were, until now, accepted as conspecific. Observations of the museum specimens and newly collected specimens, augmented by ITS2 and 28S rRNA gene fragment comparisons, indicated that S. beauforti sensu stricto infects flathead grey mullet along the eastern coast of the United States between North Carolina and Georgia. Furthermore, specimens previously identified as S. beauforti from Georgia (in part), south Florida, and the north-central Gulf of America are conspecific with Saccocoelioides macrospinosusAndrade-Gómez, Sereno-Uribe, and García-Varela, 2019, which was previously thought to infect only the Catemaco molly, Poecilia catemaconis Miller, and the white mullet, Mugil curema Valenciennes, in freshwater and brackish habitats in Veracruz, Mexico. Flathead grey mullet is a new host record for S. macrospinusus. Fish hosts and ranges for S. beauforti sensu stricto and S. macrospinosus overlap in coastal Georgia.