Juno (minor-planet designation: 3 Juno) is a large asteroid in the asteroid belt. Juno was the third asteroid discovered, in 1804, by German astronomer Karl Harding. It is tied with three other asteroids as the thirteenth largest asteroid, and it is one of the two largest stony (S-type) asteroids, along with 15 Eunomia. It is estimated to contain 1% of the total mass of the asteroid belt.

Juno was discovered on 1 September 1804, by Karl Ludwig Harding. It was the third asteroid found, but was initially considered to be a planet; it was reclassified as an asteroid and minor planet during the 1850s.

Juno is named after the mythological Juno, the highest Roman goddess. The adjectival form is Junonian (from Latin: jūnōnius), with the historical final n of the name (still seen in the French form, Junon) reappearing, analogous to Pluto: Plutonian. Juno is the international name for the asteroid, subject to local variations, such as Italian Giunone, French Junon, and Russian Юнона (Yunona).

The old astronomical symbol of Juno, still used in astrology, is a scepter topped by a star: . There are many graphic variants with more elaborate scepters, such as , sometimes tilted at an angle to provide more room for decoration. The generic asteroid symbol of a disk with its discovery number (③ for Juno) was introduced in 1852 and quickly became the norm. The scepter symbol was resurrected for astrological use in 1973.

Juno is one of the larger asteroids, perhaps tenth by size and containing approximately 1% the mass of the entire asteroid belt. It is the second-most-massive S-type asteroid after 15 Eunomia. Even so, Juno has only 3% the mass of the largest asteroid, Ceres. The orbital period of Juno is 4.36578 years.

Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties. This high albedo explains its relatively high apparent magnitude for a small object not near the inner edge of the asteroid belt. Juno can reach +7.5 at a favourable opposition, which is brighter than Neptune or Titan, and is the reason for it being discovered before the larger asteroids Hygiea, Europa, Davida, and Interamnia. At most oppositions, however, Juno only reaches a magnitude of around +8.7—only just visible with binoculars—and at smaller elongations a 3-inch (76 mm) telescope is required to resolve it. It is the main body in the Juno family.

Juno was originally considered a planet, along with 1 Ceres, 2 Pallas, and 4 Vesta. In 1811, Johann Schröter estimated Juno to be as large as 2290 km in diameter. All four were reclassified as asteroids as additional asteroids were discovered. Juno's small size and irregular shape preclude it from being designated a dwarf planet.

Juno orbits at a slightly closer mean distance to the Sun than Ceres or Pallas. Its orbit is moderately inclined at around 12° to the ecliptic, but has an extreme eccentricity, greater than that of Pluto. This high eccentricity brings Juno closer to the Sun at perihelion than Vesta and further out at aphelion than Ceres. It had the most eccentric orbit of any known body until 33 Polyhymnia was discovered in 1854, and of asteroids over 200 km in diameter only 324 Bamberga has a more eccentric orbit.