Ariel is the fourth-largest moon of Uranus. Ariel orbits and rotates in Uranus's equatorial plane, which is almost perpendicular to the planet's orbit, giving the moon an extreme seasonal cycle.

It was discovered on 24 October 1851 by William Lassell and named for a character in two different pieces of literature. As of 2019, much of the detailed knowledge of Ariel derives from a single flyby of Uranus performed by the space probe Voyager 2 in 1986, which managed to image around 35% of the moon's surface. There are no active plans at present to return to study the moon in more detail, although various concepts such as a Uranus Orbiter and Probe have been proposed.

After Miranda, Ariel is the second-closest of Uranus's five major rounded satellites. Among the smallest of the Solar System's 19 known spherical moons (it ranks 14th among them in diameter), it is believed to be composed of roughly equal parts ice and rocky material. Its mass is approximately equal in magnitude to Earth's hydrosphere.

Like all of Uranus's moons, Ariel probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of scarps, canyons, grabens and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.

Both Ariel and the slightly larger Uranian satellite Umbriel were discovered by William Lassell on 24 October 1851. Although William Herschel, who discovered Uranus's two largest moons Titania and Oberon in 1787, claimed to have observed four additional moons, this was never confirmed and those four objects are now thought to be spurious.

All of Uranus's moons are named after characters from the works of William Shakespeare or Alexander Pope's The Rape of the Lock. The names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell, though it is uncertain if Herschel devised the names, or if Lassell did so and then sought Herschel's permission. Ariel is named after the leading sylph in The Rape of the Lock. It is also the name of the spirit who serves Prospero in Shakespeare's The Tempest. The moon is also designated Uranus I.

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed an A (the initial of Ariel) combined with the low globe of Jérôme Lalande's Uranus symbol as the symbol of Ariel (). This symbol is not widely used.

Among Uranus's five major moons, Ariel is the second closest to the planet, orbiting at the distance of about 190,000 km. Its orbit has a small eccentricity and is inclined very little relative to the equator of Uranus. Its orbital period is around 2.5 Earth days, coincident with its rotational period. This means that one side of the moon always faces the planet; a condition known as tidal lock. Ariel's orbit lies completely inside the Uranian magnetosphere. The trailing hemispheres (those facing away from their directions of orbit) of airless satellites orbiting inside a magnetosphere like Ariel are struck by magnetospheric plasma co-rotating with the planet. This bombardment may lead to the darkening of the trailing hemispheres observed for all Uranian moons except Oberon (see below). Ariel also captures magnetospheric charged particles, producing a pronounced dip in energetic particle count near the moon's orbit observed by Voyager 2 in 1986.