Designated S/2004 N 1, this is the 14th known moon to circle the giant planet. It also appears to be the smallest moon in the Neptunian system, measuring just 20 km (12 miles) across, completing one revolution around Neptune every 23 hours.
US astronomer Mark Showalter spotted the tiny dot while studying segments of rings around Neptune.
Nasa said the moon was roughly 100 million times dimmer than the faintest star visible to the naked eye. It is so small that the Voyager spacecraft failed to spot it in 1989 when it passed close by Neptune and surveyed the planet's system of moons and rings.
Mr Showalter's method of discovery involved tracking the movement of a white fleck appearing over and over again in more than 150 photographs taken of Neptune by Hubble between 2004 and 2009.
"The moons and arcs orbit very quickly, so we had to devise a way to follow their motion in order to bring out the details of the system," Mr Showalter explained. "It's the same reason a sports photographer tracks a running athlete - the athlete stays in focus, but the background blurs."