Simple SummaryHaemosporidian parasites infect terrestrial vertebrates as well as avian species worldwide. However, for resident birds of the Rallidae family, such as White-breasted Waterhens, few data are available regarding haemosporidian infection. Therefore, this study aimed to detect haemosporidian infection in White-breasted Waterhens, microscopic and genetic examinations of blood samples were performed. Four species of haemosporidians were identified; however, the morphological features of onlyHaemoproteus gallinulaeare presented. Phylogenetic analysis of standard DNA sequences revealed a close relationship betweenHaemoproteusspecies and their classification into the subgenusParahaemoproteus. However, threePlasmodiumspecies were found to have a wide range of hosts. The phylogenetic tree of this parasite clearly showed individual clusters. For one of them, the species was not identified; however, it matched a previously reported species that was isolated from mosquitoes. Two of thePlasmodiumspecies were identified as well-known, generalist parasites that cause severe diseases in hosts:P. collidatumandP. elongatum. This study revealed the role of White-breasted Waterhens as carriers of haemosporidian parasites. Our findings provide potential information for further research.AbstractHaemosporidian parasites are vector-borne parasites infecting terrestrial vertebrates as well as avian species, such as the White-breasted Waterhen, a Gruiformes waterbird found in lowlands near wetlands and distributed throughout Thailand. However, information regarding haemosporidia infection in this species is lacking. To establish regional information, 17 blood samples were collected from White-breasted Waterhens. Four haemoparasite lineages were identified in six blood samples:Haemoproteus gallinulae,Plasmodium collidatum,Plasmodium elongatum, and an unidentifiedPlasmodiumspecies.H. gallinulaewas characterized with morphological features in White-breasted Waterhens for the first time; the morphological characteristics were consistent with previous descriptions.H. gallinulaewas more closely related toHaemoproteusspecies of Passeriformes birds than to those of Gruiformes birds. ThePlasmodiumparasites infecting these White-breasted Waterhens previously caused severe avian malaria in other host species. The unidentifiedPlasmodiumspecies had rarely been documented, although it was reported in theCulexvector and was possibly associated with specialist parasites either as host or habitat. Our findings reveal multiple haemosporidian species reflecting the role of this avian host as a carrier of haemosporidians. This study offers species records and molecular materials that may provide critical information for further targeted research into these haemosporidia.