Europa (/jʊˈroʊpə/ ^ⓘ) is the smallest and least massive of Jupiter's four Galilean moons. It is observable from Earth with common binoculars and is a planetary-mass moon, slightly smaller and less massive than Earth's Moon. Europa is an icy moon, and, of the three icy Galilean moons, the closest orbiting Jupiter. As a result, it exhibits a relatively young surface shaped by tidal heating.

Europa consists mainly of silicate rock, and potentially has an iron-nickel core. It has a very thin atmosphere composed primarily of oxygen. Europa has a pale, geologically young surface striated by light tan cracks and streaks; the surface lacks large-scale features such as mountains or craters, making the moon the smoothest known solid object in the Solar System. The apparent youth and smoothness of the surface is due to a water ocean beneath the surface, which could conceivably harbor extraterrestrial life. The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below.

Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the sea floor. This may be important in determining whether Europa could be habitable. In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers. In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated analysis of data obtained from the Galileo space probe, which orbited Jupiter from 1995 to 2003. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon. In March 2024, astronomers reported that the surface of Europa may have much less oxygen than previously inferred.

Europa was discovered independently by Simon Marius and Galileo Galilei. Marius later named it after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman Jupiter). In addition to Earth-bound telescope observations, Europa has been examined by a succession of space-probe flybys, the first occurring in the early 1970s. The Galileo mission, launched in 1989, provides the bulk of current data on Europa. No spacecraft has yet landed on Europa, although there have been several proposed exploration missions. In September 2022, the Juno spacecraft flew within around 320km (200 miles) of Europa for a more recent close-up view. The European Space Agency's Jupiter Icy Moons Explorer (Juice) is a mission to Ganymede launched on 14 April 2023, that will include two flybys of Europa. NASA's Europa Clipper was launched on 14 October 2024.

Europa, along with Jupiter's three other large moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on 8 January 1610, and possibly independently by Simon Marius. On 7 January, Galileo had observed Io and Europa together using a 20×-magnification refracting telescope at the University of Padua, but the low resolution could not separate the two objects. The following night, he saw Io and Europa for the first time as separate bodies.

The moon is the namesake of Europa, in Greek mythology the daughter of the Phoenician king of Tyre. Like all the Galilean satellites, Europa is named after a lover of Zeus, the Greek counterpart of Jupiter. Europa was courted by Zeus and became the queen of Crete. The naming scheme was suggested by Simon Marius, who attributed the proposal to Johannes Kepler:

Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...

The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century. In much of the earlier astronomical literature, Europa is simply referred to by its Roman numeral designation as Jupiter II (a system also introduced by Galileo) or as the "second satellite of Jupiter". In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The Voyager probes discovered three more inner satellites in 1979, so Europa is now counted as Jupiter's sixth satellite, though it is still referred to as Jupiter II. The adjectival form has stabilized as Europan.