Scientists have long known that Mars has water in the form of ice and certain minerals.
The discovery, which has major implications for the chances of life surviving on the Red Planet, was made by an orbiting European probe using ground-penetrating radar.
With surface temperatures as low as minus 68C, it would not exist as a liquid under normal conditions.
Scientists are eager to find signs of contemporary water, because such discoveries are key to unlocking the mystery of whether life ever formed on Mars in its ancient past, and whether it might persist today.
"On Earth, it's nearly a given that if a ground-penetrating radar spots stronger reflections from the sub-surface than from the surface of the polar ice, then you are seeing liquid water".
"Water is there. We have no more doubt", co-author Enrico Flamini, the Italian space agency's Mars Express mission manager, told a press conference.
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The tool is called the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), and was created to find subsurface water by sending radar pulses that penetrate the surface and ice caps.
"It's plausible that the water may be an extremely cold, concentrated brine, which would be pretty challenging for life", said Dr. Claire Cousins, an astrobiologist.
Billions of years ago, Mars is thought to have had oceans and rivers, much like Earth.
The concentration of salts needed to keep the water liquid could be fatal for any microbial life similar to Earth's, which means caution needs to be taken, with no immediate means of sampling the water. Primitive life may have evolved on the planet then, only to be snuffed out when the world lost most of its atmosphere and became an arid frozen desert.
"This subsurface anomaly on Mars has radar properties matching water or water-rich sediments", Roberto Orosei, the principal investigator, said in a statement released by the agency. For now, researchers will continue to investigate this lake and search for water elsewhere on Mars.
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The radar data showed that the lake contained a large amount of salt, Prof Orosei added.
Still, she said, there are microbes on Earth that have been able to adapt to environments like that.
But he pointed out that similar salty subglacial lakes in Antarctica had been found to support life.
It remains to be seen if more subsurface reservoirs of water will be found or whether the newly discovered one is some sort of quirk, Orosei said.
"There are single-celled organisms that survive in such an environment with a metabolism that makes use of the salt", said the professor.
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