How many moons does Jupiter have? Jupiter has at least 67 known moons ( http://solarsystem.nasa.gov/planets/jupiter/moons ). The largest four are called Io, Europa, Ganymede, and Callisto. These four moons are called the Galilean satellites because they were first seen in 1610 by the astronomer Galileo Galilei. Ganymede is the largest moon in the Solar System, with a diameter of 3,260 miles.