Hymenolaimus genus_authority = Gray (George Robert Gray), 1843 species = H. malacorhynchos binomial = Hymenolaimus malacorhynchos binomial_authority = (Gmelin (Johann Friedrich Gmelin), 1789) subdivision_ranks = Subspecies subdivision = * H. m. malacorhynchosSouth Island Blue Duck * H. m. hymenolaimusNorth Island Blue Duck

The Blue Duck (Hymenolaimus malacorhynchos) is a member of the duck, goose and swan family (family (biology)) Anatidae. It is the only member of the genus Hymenolaimus, placed in the shelduck subfamily TadorninaeFactdate=February 2007 after previously being considered part of the paraphyletic "perching duck" assemblage. The M?ori (Maori language) name is whio, which is an onomatopoetic rendition of the males call. The blue duck is born with a green beak for just 8 hours after birth; where it then develops to its final colour.

This 54 cm long species is an endemic (endemic (ecology)) resident breeder in New Zealand, nesting in hollow logs, small caves and other sheltered spots. It is a rare duck, holding territories on fast flowing mountain rivers.It is a powerful swimmer even in strong currents, but is reluctant to fly. It is difficult to find, but not particularly wary when located.

The blue duck is a dark slate-grey with a chestnut-flecked breast and a paler bill and eye. The pinkish-white bill has fleshy flaps of skin hanging from the sides of its tip. The males call is an aspirated whistle, and the females is a rattling growl.

This is a very localised species now threatened by predation from introduced mammals especially stoats, competition for its invertebrate food with introduced trout, and damming of mountain rivers for hydroelectric schemes. The New Zealand Department of Conservation is presently working on recovery programmes in habitats such as the Oparara River area of the West Coast, New Zealand and the Milford Track in Fiordland. .

The blue duck also features on the reverse side of the New Zealand $10 note.