Jupiter's innermost known natural satellite is Metis. It orbits inside Jupiter's main ring and is the first of the planet's four small inner moons, ahead of Adrastea, Amalthea, and Thebe.
Voyager 1 images revealed Metis in 1979, and the International Astronomical Union named it in 1983 after Metis, a Titaness from Greek mythology. Later observations by the Galileo spacecraft provided much better views of its irregular, strongly asymmetric shape. Metis is only a few dozen kilometers across and is far too small to resemble Jupiter's famous spherical Galilean moons.
Metis completes an orbit in roughly seven hours, faster than Jupiter rotates once. Because it lies within Jupiter's synchronous orbit, tidal interactions gradually make its orbit decay. The moon also helps supply dust to Jupiter's faint ring system: impacts eject surface material, some of which becomes ring particles.
The usual confusion is with Adrastea, which is the second-innermost known moon and also orbits within the main ring. Amalthea and Thebe are farther out. “Innermost” refers to orbital distance from Jupiter, not size, brightness, or discovery date.