The reclamation of tidal flats is implicated in the declines of a large number of migratory waterbird species along the East Asia-Australasian Flyway, and has resulted in the assessment of Yellow Sea tidal flats as an Endangered habitat by the IUCN. Created in their present form by large-scale reclamation, the Hwaseong Wetlands on the Yellow Sea coast of the Republic of Korea are comprised of tidal flats, a large reclamation lake, and extensive areas of rice-fields and fallow land. As part of preparation for increased protections for these wetlands, we conducted bird surveys between late June 2020 and mid-June 2021. During this period, we recorded more than 150,000 waterbirds in the wetland and concentrations of 1% or more of 25 populations of waterbird. We also recorded a total of 16 globally threatened wetland species. As at many other coastal wetlands in the Yellow Sea, tidal flat obligate waterbird species used the tidal flats for foraging; and roosted in artificial wetlands which had been created through the reclamation process. The extensive areas of rice-field and other freshwater habitats in the Hwaseong Wetlands were also internationally important in their own right, supporting globally threatened amphibians and internationally important concentrations of foraging geese and floodplain-associated waterbird species. The movements of waterbirds between foraging and roosting areas we recorded make clear that conservation of the site’s biodiversity either as a Ramsar site or within a serial World Heritage Property would require protection of all the contiguous tidal flats and also of the most biodiverse rice-field and freshwater wetland areas. As elsewhere in the coastal zone of the Republic of Korea, this would first require the support of local stakeholders and also a reduction in jurisdictional issues between various local and national decision-making bodies.