Identifying and evaluating potentially suitable tools to assess the status of cetaceans in coastal waters with high levels of anthropogenic threat represents a first step towards effective conservation management. Local ecological knowledge (LEK) can often provide more extensive information on focal species and biological resources than is available from standard ecological surveys, and is increasingly recognized as an important source of data for conservation research and management, but it has rarely been used as a tool to assess the status of cetaceans. We investigated the efficacy of using LEK from local fishers combined with stranding records to characterise the diversity and distribution of coastal cetaceans in the northern South China Sea, a region with high historical levels of cetacean abundance and diversity but which is experiencing intensifying anthropogenic pressures. Fishers were unable to identify most regionally occurring cetaceans to species level. However, we were able to determine the distributions of eight categories of cetaceans that were observed by fishers, and a previously unknown population of Indo-Pacific humpback dolphin reported from the coastal waters of Hainan that was later confirmed through boat-based surveys. The number of sightings of different cetacean categories reported by fishers has a significant positive linear relationship with independent data on numbers of stranded cetaceans, validating the accuracy of our respondent data and indicating that LEK can provide useful, quantitative information on abundance rankings of different cetacean categories.