Oyster reef ecosystems are in a degraded or collapsed state globally, having declined since the onset of unsustainable harvest practices and coastal development over the past two centuries. Increasingly, there is a need to understand pressures that have led to decline and address barriers to oyster reef recovery or restoration. Here, we describe the conservation status and risk of ecosystem collapse for the oyster reef ecosystems of Hong Kong and the Pearl River Estuary, the largest estuary delta in southern China. We assess and compare two community sub-types, the intertidal estuarine oyster ecosystem composed of several co-occurring species of Magallana and Saccostrea and the subtidal marine waters oyster ecosystem composed of species of Dendostrea, Magallana, and Ostrea. Following the protocol of the IUCN Red List of Ecosystems risk assessment we determined (i) key aspects of the ecosystem including: ecological description, biological characteristics, condition and collapse thresholds, threatening processes; (ii) previous and current extent of occurrence and current area of occupancy; and (iii) likelihood of collapse within the next 50 -100 years. We demonstrate that the oyster reef ecosystems of this region, both intertidal and subtidal, have been decimated by unsustainable harvest, rapid human population expansion, and coastal development for several centuries, leading to almost complete loss of structural oyster reefs (>99 % loss) and ecological function. Our overall assessment ranks the risk of collapse for the ecosystem as Critically Endangered, indicating an urgent need to protect remaining reef patches and implement oyster reef restoration programs over large scales.