Fish owls form a group of owls in the family Strigidae. Ketupa was proposed as generic name by René-Primevère Lesson in 1831 for fish owl species from Java and India. Results of a phylogenetic analysis of nine horned owl species indicate that Ketupa species form a monophyletic group.
Blakiston's fish owl (Bubo blakistoni) described by Henry Seebohm in 1883 is a fish owl from Japan and the Vladivostok region of Russia. It was thought to form the link between Ketupa and Bubo species, as its toes are bare of feathers like in Ketupa species, but it is much larger. Since its first description, and placement in the genus Bubo, it has repeatedly been placed in Ketupa; the 2013 phylogenetic analysis of nine horned owl species indicate that four Ketupa species including Blakiston's fish owl form a monophyletic group, and genetic testing of Blakiston's fish owl in Russia published in 2018 indicates a fairly large divergence from the genomes of typical Bubo.