The Harlequin Duck, Histrionicus histrionicus, is a small sea duck. In North America it is also known as Lords and ladies. Other names include painted duck, totem pole duck, rock duck, glacier duck, mountain duck, white-eyed diver, squeaker and blue streak.

Adult males are slate blue with chestnut sides and white markings including a white crescent at the base of the bill. Adult females are less colourful, with brownish-grey plumage and a white patch on the head around the eye. Both adults have a white ear patch.

Their breeding habitat is cold fast moving streams in north-western and north-eastern North America, Greenland, Iceland and western Russia. The nest is usually located in a well-concealed location on the ground near a stream. They are usually found near pounding surf and white water.

They are short distance migrants and most winter near rocky shorelines on the Atlantic and Pacific coasts. They are very rare vagrants to western Europe.

These birds feed by swimming under water or diving. They also dabble. They eat molluscs, crustaceans and insects. Harlequins have smooth, densely packed feathers that trap a lot of air within them. This is vital for insulating such small bodies against the chilly waters they ply. It also makes them exceptionally buoyant, making them bounce like corks after dives.

The eastern North American population is declining and is considered endangered. Possible causes include loss of habitat due to hydroelectric projects and loss of life due to oil spills near coastal areas.

Today, this is the only species of its genus. Two prehistoric harlequin ducks were described from fossils, although both were initially placed in a distinct genus: Histrionicus shotwelli is known from Middle to Late Miocene deposits of Oregon, USA and was considered to form a distinct monotypic genus, Ocyplonessa. Histrionicus ceruttii which lived in California during the Late Pliocene was at first taken to be a species of the related genus Melanitta.

The Harlequin Duck takes its name from Arlecchino, Harlequin in French, a colourfully dressed character in Commedia dell'arte. The species name comes from the Latin word "histrio", "actor".