Dione (/daɪˈoʊni/) is the fourth-largest moon of Saturn. With a mean diameter of 1,123 km and a density of about 1.48 g/cm³, Dione is composed of an icy mantle and crust overlying a silicate rocky core, with rock and water ice roughly equal in mass. Its trailing hemisphere is marked by large cliffs and scarps called chasmata; the trailing hemisphere is also significantly darker compared to the leading hemisphere.

The moon was discovered by Italian astronomer Giovanni Domenico Cassini in 1684 and is named after the Titaness Dione in Greek mythology. Dione was first imaged up-close by the Voyager 1 space probe in 1980. Later, the Cassini spacecraft made multiple flybys of Dione throughout the 2000s and 2010s as part of its campaign to explore the Saturn system.

Giovanni Domenico Cassini named the four moons he discovered (Tethys, Dione, Rhea, and Iapetus) Sidera Lodoicea ("the stars of Louis") to honor king Louis XIV. Cassini found Dione in 1684 using a large aerial telescope he set up on the grounds of the Paris Observatory. The satellites of Saturn were not named until 1847, when William Herschel's son John Herschel published Results of Astronomical Observations made at the Cape of Good Hope, suggesting that the names of the Titans (sisters and brothers of Cronus) be used.

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 a Greek delta (the initial of Dione) combined with the crook of the Saturn symbol as the symbol of Dione (). This symbol is not widely used.

Dione orbits Saturn with a semimajor axis about 2% less than that of Earth's Moon. However, reflecting Saturn's greater mass (95 times that of Earth), Dione's orbital period is one tenth that of Earth's Moon. Dione is currently in a 1:2 mean-motion orbital resonance with moon Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus. This resonance maintains Enceladus's orbital eccentricity (0.0047), providing a source of heat for Enceladus's extensive geological activity, which shows up most dramatically in its cryovolcanic geyser-like jets. The resonance also maintains a smaller eccentricity in Dione's orbit (0.0022), tidally heating it as well.

Dione has two co-orbital, or trojan, moons, Helene and Polydeuces. They are located within Dione's Lagrangian points L₄ and L₅, 60 degrees ahead of and behind Dione respectively. A leading co-orbital moon twelve degrees ahead of Helene was reported by Stephen P. Synnott in 1982.

At 1,122 km (697 mi) in diameter, Dione is the 15th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined. It is also Saturn's fourth-largest moon. Based on its density, Dione's interior is likely a combination of silicate rock and water ice in nearly equal parts by mass.

Shape and gravity observations collected by Cassini suggest a roughly 400 km radius rocky core surrounded by a roughly 160 km envelope of H₂O, mainly in the form of water ice, but with some models suggesting that the lowermost part of this layer could be in the form of an internal liquid salt water ocean (a situation similar to that of its orbital resonance partner, Enceladus). Downward bending of the surface associated with the 1.5 km high ridge Janiculum Dorsa can most easily be explained by the presence of such an ocean. Neither moon has a shape close to hydrostatic equilibrium; the deviations are maintained by isostasy. Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, where tidal heating of the crust is greatest.