Digenean trematodes are a diverse group of parasites that require multiple host organisms to complete their life cycles. In the east coast of North America, nine species of trematodes infect the eastern mudsnail Tritia obsoleta (TO) as a first-intermediate host. Four species of these trematodes infect fish as their definitive hosts. Estuarine habitats of the east coast have also been colonized by red alga Agarophyton vermiculophyllum (AV), which provides nursery, refuge, and shelter for native macroinvertebrates, including TO. The arrival of this non-native macroalga changed abundance and diversity of macroinvertebrates, and subsequently the changes in these macroinvertebrates’ parasites. To understand the biogeographic pattern of fish-infecting trematodes in AV-colonized estuaries, we sampled sixteen estuarine habitats along the US east coast from Charleston, SC to Durham, NH in summer 2019. Per site, we recorded AV biomass, TO density, and environmental variables (e.g., temperature, salinity, dissolved oxygen), and randomly collected 100 TO for parasite dissections. Overall, we found 11.75% infection prevalence across all TO (n = 1,600). Of those infected, 57% were infected by trematodes using fish as definitive hosts. Using GLM and AIC, we found that the biomass of AV was one of the best variables predicting parasite diversity measures (e.g. richness, diversity index, prevalence). The transformation of estuaries by AV can change population structures of fish and macroinvertebrates that are obligatory hosts for these trematodes. Thus, trematode diversity can be used as a proxy to determine and forecast population dynamics of hosts and potentially fisheries in these rapidly changing estuaries.