The king shag is a large black-and-white shag with big pink feet. The black feathers on forehead, crown and nape extend to just under the bill, making king shags appear ‘dark-headed’ compared to the similar-sized pied shag. The dorsal area of mantle, scapulars, back and tail are black apart from adults, which have a white patch or stripe near the leading edge of the inner wing. Some birds also have a white patch on the upper back. The white scapular feathers are prominent on roosting birds, creating an elongated white patch in the middle of the otherwise black folded wing. Breeding adults have a patch of sulphur-yellow warty ‘caruncles’ on each side of the upper bill base; the size and colour intensity of these is reduced at other times of the year. The cobalt blue eye-ring (not part of the eye) is shared with other pink-footed ‘subantarctic’ shags, providing this group with the generic, but technically incorrect name “blue-eyed shags”. Juvenile birds differ from the adults in having dull brown upper parts (rather than black) and pale facial skin.

Voice: croaking bursts of low frequency sounds, more noticeable during the early morning.

Similar species: pied shags are similar in size, but have a predominantly white face, with the eye surrounded by white feathers; they also lack any white feathers on the upper wings and back. Pied shags are also slimmer, compared to the thick-necked look of king shags. Pied morph Stewart Island shags are very similar, but are not known to overlap in range. They differ in having a more slender bill and orange (cf. yellow) caruncles when breeding.