In astronomy, a regular moon or a regular satellite is a natural satellite following a relatively close, stable, and circular orbit which is generally aligned to its primary's equator. They form within discs of debris and gas that once surrounded their primary, usually the aftermath of a large collision or leftover material accumulated from the protoplanetary disc. Young regular moons then begin to accumulate material within the circumplanetary disc in a process similar to planetary accretion, as opposed to irregular moons, which formed independently before being captured into orbit around the primary.

Regular moons are extremely diverse in their physical characteristics. The largest regular moons are massive enough to be gravitationally rounded, with two regular moons—Ganymede and Titan—being larger than the planet Mercury. Large regular moons also support varied and complex geology. Several are known to have atmospheres, although only one regular moon—Titan—hosts a significant atmosphere capable of supporting weather and climate. As a result of their complexity, the rounded regular moons are often considered planetary objects in their own right by planetary scientists. In contrast, the smallest regular moons lack active geology. Most are heavily cratered and irregular in shape, often resembling small asteroids and other minor bodies in appearance.

Six of the eight planets of the Solar System host 60 regular satellites combined, with the four giant planets—Jupiter, Saturn, Uranus, and Neptune—hosting the most extensive and complex regular satellite systems. At least four of the nine likeliest dwarf planets also host regular moon systems: Pluto, Eris, Haumea, and Orcus.

Regular moons have several different formation mechanisms. The regular moons of the giant planets are generally believed to have formed from accreting material within circumplanetary discs, growing progressively from smaller moonlets in a manner similar to the formation of planets. Multiple generations of regular satellite systems may have formed around the giant planets before interactions with the circumplanetary disc and with each other resulted in inward spiralling into the parent planet. As gas inflow into the parent planet begins to end, the effects of gas-induced migration decrease, allowing for a final generation of moons to survive.

In contrast, Earth's Moon and Pluto's five satellites are thought to have originated from giant impacts between two protoplanets early in the Solar System's history. These impacts ejected a dense disc of debris into orbit whence satellites can accrete. The giant impact model has also been applied to explain the origin of other dwarf planet satellite systems, including Eris's moon Dysnomia, Orcus's moon Vanth, and Haumea's ring and two moons. In contrast to regular moon systems of the giant planets, giant impacts can give rise to unusually massive satellites; Charon's mass ratio to Pluto is roughly 0.12.

Regular moons may also originate from secondary disruption events, being fragments of other regular moons following collisions or due to tidal disruption. The regular moons of Neptune are likely examples of this, as the capture of Neptune's largest moon—Triton—would have severely disrupted the existing primordial moon system. Once Triton was tidally dampened into a lower-eccentricity orbit, the debris resulting from the disruption of the primordial moons re-accreted into the current regular moons of Neptune.

Despite the extensive exploration of Mars, the origin of Mars's two moons remains the subject of ongoing debate. Phobos and Deimos were originally proposed to be captured asteroids originating from the neighboring asteroid belt, and thus would not be classified as regular satellites. Their similarities to C-type asteroids with respect to spectra, density, and albedo further supported this model.

However, the capture model may be inconsistent with the small, low-eccentricity, low-inclination orbits of the two moons, which are more typical of regular satellites. The rubble pile nature of Phobos has further pointed against a captured origin, and infrared observations of Deimos by the Hope orbiter have revealed that the moon's surface is basaltic in composition, more consistent with an origin around Mars. As a result, various models for the in situ formation of Phobos and Deimos have been proposed to better explain their origins and current configuration, including a giant impact scenario similar to the one which formed the Moon and a 'recycling' model for Phobos.