The king cobra was described and drawn by the Danish naturalist Theodore Edward Cantor in 1836, who gave it the scientific name Hamadryas hannah. Cantor had three specimens from the Sundarbans and one caught in the vicinity of Kolkata. It was subordinated to the genus Ophiophagus by Albert Günther in 1864.
Ophiophagus hannah belongs to the monotypic genus Ophiophagus in the family Elapidae, while most other cobras are members of the genus Naja. They can be distinguished from other cobras by size and hood. King cobras are generally larger than other cobras, and the stripe on the neck is a chevron instead of a double or single eye shape that may be seen in most of the other Asian cobras. Moreover, the hood of the king cobra is narrower and longer. A key to identification, clearly visible on the head, is the presence of a pair of large scales known as occipitals, located at the back of the top of the head. These are behind the usual "nine-plate" arrangement typical of colubrids and elapids, and are unique to the king cobra.
The skin of king cobra is dark olive or brown with black bands and white or yellow crossbands. The head is black with two crossbars near the snout and two behind the eyes. Adult king cobras are 3.18 to 4 m (10.4 to 13.1 ft) long. The longest known individual measured 5.85 m (19.2 ft). Its belly is cream or pale yellow. It has 17 to 19 rows of smooth scales. Ventral scales are uniformly oval shaped. Dorsal scales are placed in an oblique arrangement. Males have 235 to 250 ventral scales, while females have 239 to 265. The subcaudal scales are single or paired in each row, numbering 83 to 96 in males and 77 to 98 in females.
Juveniles are shiny black with narrow yellow bands (can be mistaken for a banded krait, but readily identified with its expandable hood). The head of a mature snake can be quite massive and bulky in appearance, though like all snakes, it can expand its jaws to swallow large prey items. It has proteroglyph dentition, meaning it has two short, fixed fangs in the front of the mouth, which channel venom into the prey like hypodermic needles. The average lifespan of a wild king cobra is about 20 years.