Gnarled Mossy Cloud Forest is a globally unique ecosystem, combining floristic elements from Australia, New Zealand and New Caledonia. It is restricted to a very small area (28ha) at elevations above 750m on the summits of two mountains on Lord Howe Island in the Pacific Ocean, approximately 570km off the east coast of Australia. Moisture derived from clouds is a key feature of the ecosystem. We assessed the conservation status of this ecosystem using the International Union for the Conservation of Nature Red List criteria for ecosystems. There has been no historical clearing of the ecosystem, but declines (with large uncertainty bounds) were estimated for two abiotic variables that are important in maintaining the component species (cloud cover and rainfall). Overall, we found the ecosystem to be Critically Endangered based on a restricted geographic distribution combined with continuing decline (criterion B1aii, iii, B1b, B1c and B2aii, iii, B2b, B2c). Decline was inferred from: a loss of moisture from declining rainfall and cloud cover due to climate change (affecting disturbance regimes, gap formation and species survival and recruitment); ongoing exotic rat predation on seeds and seedlings of several sensitive species that are structural components of the ecosystem (affecting survival and recruitment); and the fact that the ecosystem is considered to exist at only one location. This mirrors similar threats from exotic species and climate change to other Pacific island cloud forests. Eradication of rats from Lord Howe Island will reduce the immediate risk to this ecosystem; however, only global mitigation of greenhouse gases could alleviate risk from declining cloud cover and moisture availability.