While observing the new "planet" from Hawaii last month, a team of astronomers led by Michael Brown of the California Institute of Technology spotted a faint object trailing next to it.
Because it was moving, astronomers ruled it was a moon and not a background star, which is stationary.
The moon discovery is important because it can help scientists determine the new planet's mass.
In July, Brown announced the discovery of an icy, rocky object larger than Pluto in the Kuiper Belt, a disc of icy bodies beyond Neptune.
Brown labelled the object a planet and nicknamed it Xena after the lead character in the former TV series Xena: Warrior Princess.
Definition of planet
By determining the moon's distance and orbit around Xena, scientists can calculate how heavy Xena is. The faster a moon goes around a planet, the more massive a planet is.
But the newly discovered moon, nicknamed Gabrielle after Xena's faithful travelling sidekick in the TV series, probably will not quell the debate over what exactly is a planet and whether Pluto should keep its status.
The problem is there is no official definition for a planet, and setting standards such as size limits potentially invites other objects to take the "planet" label.
Possessing a moon is not a criteria of planethood since Mercury and Venus are moonless planets.
Brown said he had expected to find a moon orbiting Xena because many Kuiper Belt objects are paired with moons.
The moon is about 250km wide and 60 times fainter than Xena, the farthest-known object in the solar system. It is 14.5 billion km away from the sun, or about three times Pluto's distance from the sun.
Scientists believe Xena's moon was formed when Kuiper Belt objects collided with one another. The Earth's moon is said to have been formed in a similar way, when the Earth crashed into an object the size of Mars.
Xena's moon was first spotted by a 10-metre telescope at the Keck Observatory in Hawaii on 10 September.
Scientists expect to learn more about the moon's composition during observations with the Hubble Space Telescope in November.
Brown planned to submit a paper describing the moon discovery to the Astrophysical Journal next week.
The International Astronomical Union, a group of scientists responsible for naming planets, is deciding on formal names for Xena and Gabrielle.