Klaas’s cuckoo is 16–18 cm in length. The species exhibits sexual dimorphism. Males have a glossy green body with few markings and plain white underparts. Females have a bronze-brown body, greenish wing coverts and faintly barred white underparts. Viewed in flight, the male is largely white with dark primaries and females appear mostly brown. Males and females both have a small white post-ocular patch.