The rock ball was photographed on Sept. 11 while the U.S. space agency's Curiosity rover was exploring a region of Mars known as the Gale Crater.
One of Curiosity's cameras captured several images of the centimetre-wide ball as part of the stream of photographs it regularly takes of its surroundings and beams back to Earth.
It's not the first time rocky spheres have been found on Mars. In 2004, NASA's Opportunity rover photographed a group of tiny balls made of a ferrous mineral called hematite. Opportunity photographed still more spheres, of a different composition, eight years later.
The spheres likely formed through a process called concretion, where minerals precipitate within sedimentary rock, often into oval or spherical shapes. Similar formations can be found in many places on Earth.
If in fact concretion caused the Mars spheres, then they would be evidence there was once water on the planet. However, some scientists believe the rock balls might be the leftovers from meteorites that broke up in the Martian atmosphere.
Measuring roughly three by three metres, the nearly one-tonne Curiosity landed on Mars in 2012 and has been exploring the planet's surface ever since. It is the fourth successful rover to explore the Red Planet.