Based on surveys during 2000–2012 and best available knowledge, we estimate the global population size of the Endangered Scaly-sided Merganser Mergus squamatus to be c.1,940 pairs (or c.4,660 birds prior to reproduction). In Russia, surveys identified the breeding range in the Sikhote-Alin mountain range and the area adjoining the lower Amur River. Particular effort was made to define the edge of the range. Breeding densities in the Sikhote-Alin were applied to un-surveyed rivers within the range to estimate overall numbers. The breeding range comprises c.7,800 km of 120 rivers: 6,800 km in Russia, 600 in China and 400 in DPR Korea. In Russia it comprises 88 rivers of both the western and eastern slopes of the Sikhote-Alin Range, and two small isolated areas, one in central Khabarovsk and the other in the Jewish Autonomous Oblast. All known and potential breeding areas were surveyed in China, where the breeding range comprises the western slope of the Changbai Mountains, and one isolated river system in the Lesser Xingan Mountains. Scaly-sided Mergansers are also assumed to breed on the eastern slope of the Changbai Mountains in DPR Korea. No recent surveys have been made there but numbers were estimated by applying densities from China to rivers with apparently suitable habitat. Detailed examination of past records suggests that an earlier claim of breeding Scaly-sided Mergansers on the Dep River was erroneous and we conclude that the breeding range never extended as far west as has been portrayed in most literature. The lack of comparable surveys previously precludes an assessment of change in population range or size, though it seems inevitable that habitat loss in China is likely to have resulted in some loss of range and numbers, particularly in Heilongjiang Province, in recent decades.