Ganymede (or Jupiter III) is the largest and most massive natural satellite of Jupiter as well as in the Solar System, being a planetary-mass moon. It is the largest Solar System object without an atmosphere, despite being the only moon of the Solar System with a magnetic field. Like Titan it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io or the Moon.

Ganymede is composed of approximately equal amounts of silicate rock and water. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of it. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity due to tidal heating. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System. Ganymede's magnetic field is probably created by convection within its liquid iron core, also created by Jupiter's tidal forces. The meager magnetic field is buried within Jupiter's far larger magnetic field and would show only as a local perturbation of the field lines. Ganymede has a thin oxygen atmosphere that includes O, O₂, and possibly O₃ (ozone). Atomic hydrogen is a minor atmospheric constituent. Whether Ganymede has an ionosphere associated with its atmosphere is unresolved.

Ganymede's discovery is credited to Galileo Galilei, the first to observe it, on January 7, 1610, as the third of the Galilean moons, the first group of objects discovered orbiting another planet. Its name was soon suggested by astronomer Simon Marius, after the mythological Ganymede, a Trojan prince desired by Zeus (the Greek counterpart of Jupiter), who carried him off to be the cupbearer of the gods. Beginning with Pioneer 10, several spacecraft have explored Ganymede. The Voyager probes, Voyager 1 and Voyager 2, refined measurements of its size, while Galileo discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), which was launched in 2023. After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede.