Planorbarius corneus, common name the great ramshorn, is a relatively large species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails. It is native to Europe, though is most common in central Europe and the Balkans region. This species possesses the protein hemoglobin in its blood, which allows it to live in oxygen-poor environments. It may live for up to 6 years in captivity, and likely has a reproductive cycle of about 1 year.

The shell of this species appears to be dextral in coiling, even though it is in fact sinistral or left-handed.^{[citation needed]}

Planorbarius corneus is known by a long list of taxonomic synonyms, the earliest of which was Helix cornea in 1758. Carl Linnaeus was the first to describe the species in the 10th edition of the Systema Naturae.

This species formerly had 4 subspecies; Planorbarius corneus arabatzis, Planorbarius corneus corneus, Planorbarius corneus etruscus, and Planorbarius corneus grandis. P. arabatzis and P. grandis are now considered distinct species, while the other two are no longer valid as subspecies.

Planorbarius corneus is distributed from western Europe, through central Europe and into the Caucasus, north into Siberia and south into the Middle East. Additionally, as the species is commonly sold in the aquarium trade, it may be found outside of its main range in ponds where they have been introduced. The species is very common in central Europe and the Balkans, where it has been recorded at densities of up to 40 individuals per square meter. It is not as common in southwestern Europe.

In western Europe, it has been recorded in Belgium, France and the British Isles (including Britain, Ireland, the Isle of Man, Guernsey and Jersey). It is not found in Spain, but it has been recorded on some Spanish and Portuguese islands, including Madeira, the Azores, the Canary Islands. In the Nordic countries, it has been recorded in Denmark, Finland, Sweden and Norway. In central Europe, it is found in Austria, Germany, the Netherlands, Luxembourg, Lichtenstein, Switzerland and the Czech Republic, and in southern Europe it has been recorded in Greece and Italy. In eastern Europe and the Caucasus, its range includes Albania, Armenia, Azerbaijan, Bosnia and Herzegovina, Bulgaria, Croatia, Estonia, Hungary, Latvia, Lithuania, North Macedonia, Montenegro, Poland, Romania, Serbia, Slovakia, Slovenia and Ukraine. The species is also found in western Asia, having been recorded in Kazakhstan, Iran, western regions of Russia, Turkey, Turkmenistan and Uzbekistan.

Planorbarius corneus is the largest European species of ramshorn snail (family Planorbidae), with a shell typically measuring 35 mm (1.4 in) in diameter and 12 mm (0.47 in) in width when fully-grown. The shell is a reddish-brown or olive color, though albino variants may have lighter colors. The shell is somewhat reflective, with visible growth lines, and juvenile snails possess hairs along the shell's surface. There are between 5–5.5 whorls, or revolutions of the shell. Each whorl is round, with no observable keel. Each successive whorl grows in size, leaving the center of the shell (the umbilicus) sunken. Sutures between whorls are deep. The opening of the shell, called the aperture, is kidney-shaped. The animal's skin can be brown or black in coloration, but albino variants may appear red or white. A protrusible siphon is present near the front edge of the shell, connecting to a chamber in which the lungs reside.

The 10–17 by 22–40 millimetres (0.39–0.67 by 0.87–1.57 in) coiled shell has between 3 and 4.5 rounded whorls with deep sutures, the last whorl predominating. The upper side is weakly depressed and the lower side is deeply depressed (flattened on the underside but spire recessed on the upper side). There is no keel. The shell is light yellowish with a brown, reddish or greenish periostracum, radially and spirally weakly striated. The aperture is wide and almost circular. All species within family Planorbidae have sinistral shells.^{[citation needed]}