What is the largest asteroid? The largest asteroid is called Ceres. It is about one-quarter the size of the moon and orbits the sun between Mars and Jupiter in a region called the asteroid belt. Unlike most asteroids, Ceres is spherical in shape. Ceres was discovered by the Italian astronomer Giuseppe Piazzi in 1801 as he searched for a planet which was predicted to exist between Mars and Jupiter. It was the first asteroid ever discovered.