Liquid ethane is a component of crude oil.
Cassini has made more than 40 close flybys of Titan, a giant planet-sized satellite of the ringed world.
Scientists had theorized that Titan might have oceans of methane, ethane and other hydrocarbons, but Cassini found hundreds of dark, lake-like features instead, and it wasn't known at first whether they were liquid or dark, solid material, JPL's statement said.
"This is the first observation that really pins down that Titan has a surface lake filled with liquid," Bob Brown, team leader of Cassini's visual and mapping instrument, said in the statement.
The instrument was used during a December flyby to observe a feature dubbed Ontario Lacus, in the south polar region, that is about 7,800 square miles, slightly larger than North America's Lake Ontario.
Cassini reached Saturn in mid-2004 and at the end of that year launched a probe named Huygens that parachuted to the surface of Titan the following January.