With new information from a recent mtDNA study, it appears that the Himalayan cuckoo is most closely related to Cuculus canorus, creating a sister-clade with the two other species: C.gularis and C.rochii. The species C.optatus, previously named horsfieldi for Palearctic populations but was proven unsuitable,  is considered a separate species because their songs were determined distinct enough.
Recent mtDNA research suggests that the Himalayan cuckoo is most closely related to the common cuckoo (Cuculus canorus), forming a sister-clade with the other two species: C.gularis and C.rochii. The previously named species C.optatus, which was originally designated for Palearctic populations but later deemed unsuitable, is now considered a distinct species due to its unique song characteristics.