The moon, provisionally named S/2012 (134340) 1, or P5, travels in an orbit around Pluto that is 95,000 kilometres in diameter and lies in the same plane as the orbits of its other moons.
"The moons form a series of neatly nested orbits, a bit like Russian dolls," said Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, Calif., who led the team that discovered the new moon.
P5 is irregular in shape and measures 10 to 25 kilometres across. Its orbit is the second closest to the dwarf planet, next to the largest moon, Charon, which was discovered in 1978 and measures about 1,000 kilometres across.
Astronomers have speculated that for every circuit around Pluto that P5 makes, Charon likely makes three.
A speck in Hubble's eye
P5 was detected as a speck of light on nine images taken at the end of June and early July by Hubble's wide field camera.
The last Pluto moon Hubble spotted was P4 in 2011. Five years prior to that, the telescope detected two other small moons, known as Nix and Hydra.
"The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system," said Harold Weaver of the Applied Physics Laboratory at Johns Hopkins University in Laurel, Md., in a NASA news release.
Weaver was part of the team that found P5. The team also included S. Alan Stern, Andrew J. Steffl and Marc W. Buie of the Southwest Research Institute in San Antonio, Texas.
Astronomers suspect Pluto's many moons resulted from a collision between the dwarf planet and another object in the Kuiper belt, which is the region at the outer edge of the solar system where Pluto and other small icy objects reside.
Pluto, which is smaller than the Earth's moon, was demoted from a planet to a dwarf planet in 2006 after the International Astronomical Union introduced new guidelines for classifying celestial bodies.
Astronomers hope to learn even more about Pluto and its moons when the New Horizons spacecraft makes a high-speed fly-by past the distant dwarf planet in July 2015, getting as close as 10,000 kilometres from Pluto.
Scientists hope the NASA spacecraft will bring back the first detailed images of Pluto, which is so small and far away that even the Hubble can only barely detect very large features on its surface.