The common moorhen (Gallinula chloropus), also known as the waterhen, is a bird species in the rail family (Rallidae). It is distributed across many parts of the Old World, across Africa, Europe, and Asia. It lives around well-vegetated marshes, ponds, canals and other wetlands. The species is not found in the polar regions or many tropical rainforests; generally it is one of the most common Old World rail species, together with the Eurasian coot in some regions.

The common moorhen was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae. He placed it in the genus Fulica and coined the binomial name Fulica chloropus. The common moorhen is now one of five extant species placed in the genus Gallinula that was introduced in 1760 by the French zoologist Mathurin Jacques Brisson. The genus name is from Latin gallinula meaning "little hen" or "little chicken". The specific epithet chloropus combines the Ancient Greek khlōros χλωρός meaning "green" and pous (πούς) meaning "foot".

The closely related common gallinule G. galeata of the New World, and the tristan moorhen G. nesiotis and gough moorhen G. comeri of the Tristan da Cunha archipelago, formerly often regarded as conspecific, are now treated as a separate species by all the ornithological authorities, following the discovery of significant genetic differences in addition to differences in the structure of the red bill shield and vocal differences. The final species in the genus, the dusky moorhen G. tenebrosa of Australasia, has also been considered conspecific by some authors in the past.

The name mor-hen has been recorded in English since the 13th century. The word moor here is in its old sense meaning marsh; the species is not usually found in what is now called moorland. Another old name, waterhen, is more descriptive of the bird's habitat. A "watercock" is not a male "waterhen" but the rail species Gallicrex cinerea, not closely related to the common moorhen. "Water rail" usually refers to Rallus aquaticus, again not closely related.

Five subspecies are currently accepted:

The moorhen is a distinctive species, with predominantly black and brown plumage, with the exception of a white under-tail, white streaks on the flanks, yellow legs and a red frontal shield. The bill is red with a yellow tip. The young are browner and lack the red shield. The frontal shield of the adult has a rounded top and fairly parallel sides; the tailward margin of the red unfeathered area is a smooth waving line. The subspecies G. c. meridionalis is smaller than the nominate, has slaty blue-grey upperwing coverts and lacks the olive wash. Subspecies G. c. orientalis is similar to G. c. meridionalis but has a larger shield. Subspecies G. c. pyrrhorrhoa is darker than the nominate; subspecies G. c. pyrrhorrhoa has buff undertail coverts. In the related common gallinule (Gallinula galeata) of the Americas, the frontal shield has a fairly straight top and is less wide towards the bill, giving a marked indentation to the back margin of the red area.

The common moorhen gives a wide range of gargling calls and will emit loud hisses when threatened. A midsized to large rail, it can range from 30 to 38 cm (12 to 15 in) in length and span 50 to 62 cm (20 to 24 in) across the wings. The body mass of this species can range from 192 to 500 g (6+3⁄4 to 17+3⁄4 oz).

This is a common breeding and resident bird in marsh environments, rivers, well-vegetated lakes and even in city parks. Populations in areas where the waters freeze, such as eastern Europe, migrate to more temperate climates. In China, common moorhen populations are largely resident south of the Yangtze River, whilst northern populations migrate in the winter; these populations show high genetic diversity.