An excerpt from the BBC: What they believe to be a lake sits under the planet's south polar ice cap, and is about 20km (12 miles) across.
Previous research found possible signs of intermittent liquid water flowing on the martian surface, but this is the first sign of a persistent body of water on the planet in the present day.
Lake beds like those explored by Nasa's Curiosity rover show water was present on the surface of Mars in the past.
However, the planet's climate has since cooled due to its thin atmosphere, leaving most of its water locked up in ice.
The result is exciting because scientists have long searched for signs of present-day liquid water on Mars, but these have come up empty or yielded ambiguous findings. It will also interest those studying the possibilities for life beyond Earth - though it does not yet raise the stakes in the search for biology.
The discovery was made using Marsis, a radar instrument on board the European Space Agency's (Esa) Mars Express orbiter.