Bycatch mortality is a significant driver of marine mammal population declines. However, there is little information available on patterns or magnitude of bycatch mortality in many heavily fished Asian marine systems such as the South China Sea (SCS). To address this limited knowledge base, we conducted interviews with fishers to gather local ecological knowledge on marine mammal bycatch around Hainan Island, China. Gillnets were the primary fishing gear used in local fisheries, and were also responsible for the majority of reported marine mammal bycatch events in recent decades. Bycatch events were reported from all seasons but were most frequent in spring (38.4%), which might relate to seasonal variation in fishing activities. The spatial pattern of relative bycatch densities for Indo-Pacific humpback dolphins, Indo-Pacific finless porpoises and unidentified small dolphins varied around Hainan and neighbouring waters. A substantial proportion of informants (36.1 and 9.2% respectively) reported that they have eaten or sold marine mammal meat, demonstrating the continued existence of cultural practices of consuming marine mammals on Hainan. Responses of fishers to bycatch events were dependent both on their existing attitudes and perceptions towards marine mammals and on other sociocultural factors. Almost half of informants agreed that marine mammal populations in the SCS have decreased. Declines were thought by informants to have been caused by overfishing, water pollution and vessel collisions, with bycatch responsible for further declines in dolphins.