Iapetus (/aɪˈæpətəs/) is the outermost of Saturn's large moons. With an estimated diameter of 1,469 km (913 mi), it is the third-largest moon of Saturn and the eleventh-largest in the Solar System. Named after the Titan Iapetus from Greek mythology, the moon was discovered in 1671 by Giovanni Domenico Cassini.

A relatively low-density body composed mostly of ice, Iapetus is home to several distinctive and unusual features, such as a striking difference in coloration between its dark leading hemisphere and its bright trailing hemisphere, as well as a massive equatorial ridge that runs three-quarters of the way around the moon.

Iapetus was discovered by Giovanni Domenico Cassini, an Italian-born French astronomer, in October 1671. It was the first moon that Cassini discovered; the second moon of Saturn to be discovered after Christiaan Huygens spotted Titan 16 years prior in 1655; and the sixth extraterrestrial moon to be discovered in human history.

Cassini discovered Iapetus when the moon was on the western side of Saturn, but when he tried viewing it on the eastern side some months later, he was unsuccessful. This was also the case the following year, when he was again able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side.

Iapetus is named after the Titan Iapetus from Greek mythology. The name was suggested by John Herschel (son of William Herschel) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, in which he advocated naming the moons of Saturn after the Titans, brothers and sisters of the Titan Cronus (whom the Romans equated with their god Saturn); and Giants, the massive but lesser relatives of the Titans who sided with the Titans against Zeus and the Olympian Gods.

The name has a largely obsolete variant, Japetus /ˈdʒæpɪtəs/, with an adjectival form Japetian. These occurred because there was no distinction between the letters ⟨i⟩ and ⟨j⟩ in Latin, and authors rendered them differently.

When first discovered, Iapetus was among the four Saturnian moons labelled the Sidera Lodoicea by their discoverer Giovanni Cassini after King Louis XIV (the other three were Tethys, Dione and Rhea). However, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being Saturn V because it was the fifth known Saturnian moon in order of distance from Saturn at that time. Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended and Iapetus became Saturn VII. With the discovery of Hyperion in 1848, Iapetus became Saturn VIII, which is still its Roman numerical designation today. Geological features on Iapetus are generally named after characters and places from the French epic poem The Song of Roland.