The tiny new moon, with an estimated diameter of just 8 to 12 miles, has been temporarily designated P4 and joins the moons Charon, the largest at 648 miles across, and Nix and Hydra, both about 20 to 70 miles wide.
The new moon was discovered as Hubble searched for rings around Pluto, which has a diameter of 1,441 miles, a NASA release said Wednesday.
"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles," said Mark Showalter of the SETI Institute in Mountain View, Calif., who led the observing program.
The ring survey was part of preparations for NASA's New Horizons mission, with a space probe scheduled to fly through the Pluto system in 2015.
The mission will bring new insights about worlds at the edge of our solar system, NASA said.
The new P4 moon is located between the orbits of Nix and Hydra which Hubble discovered in 2005.
"This surprising observation is a powerful reminder of Hubble's ability as a general purpose astronomical observatory to make astounding, unintended discoveries," said Jon Morse, astrophysics division director at NASA Headquarters in Washington.
Charon was discovered in 1978 at the U.S. Naval Observatory, while Pluto was discovered in 1930 by Clyde Tombaugh at Lowell Observatory in Flagstaff, Ariz.