How many moons does Neptune have? Neptune has thirteen moons that we know of. The largest moon is Triton (shown to the left). Triton is slightly smaller than Earth's moon and has active volcanoes which erupt like geysers and eject nitrogen frost over the surface. The other moons of Neptune are Nereid, Proteus, Larissa, Despina, Galatea, Thalassa, and Naiad. These moons are much smaller than Triton and, except for Nereid, were not discovered until the Voyager spacecraft flew by them in 1989.