The space agency says the rover, called Opportunity, has come across sulfates and other minerals that form in the presence of water. The discovery was made during a study of fine, layered rock.
NASA scientists say the robot hasn't found any direct traces of living organisms. However, they say the findings suggest that if life had been present when the rocks were formed, then the living conditions could have permitted an organism to flourish.
Opportunity and its twin rover, Spirit, are being controlled by scientists working at California's Jet Propulsion Laboratory.
One principal investigator says Opportunity's study of the formations shows liquid water once flowed on Mars and changed the chemistry and composition of the rocks.