The plaintive cuckoo (Cacomantis merulinus) is a species of bird belonging to the genus Cacomantis in the cuckoo family Cuculidae. It is native to Asia, from India, Nepal and China to Indonesia. There are four subspecies. The nominate form C. m. merulinus is found in the Philippines where it is common on many of the larger islands. C. m. querulus is the most widespread form, occurring in north-east India, Bangladesh, southern China, Indonesia, Myanmar, Thailand, Cambodia, Laos and Vietnam. It is a summer visitor to most of its Chinese range, migrating south for the winter. C. m. threnodes is found in the Malay Peninsula, Sumatra and Borneo while C. m. lanceolatus occurs in Java, Bali and Sulawesi.

The grey-bellied cuckoo (C. passerinus) was formerly classified as a subspecies of the plaintive cuckoo but is now often treated as a separate species.