The river valley on Titan stretches more than 400 kilometres from its "headwaters" to a large sea, a NASA Jet Propulsion Laboratory statement said.
In comparison, the Nile river on Earth stretches about 6,700 kilometres.
Images by NASA's Cassini mission have revealed for the first time a river system this vast and in such high resolution anywhere other than Earth.
Titan is known to have vast seas - the only other body in the solar system, apart from Earth, to possess a cycle of liquids on its surface.
However, the thick Titan atmosphere is a frigid one, meaning liquid water couldn't possibly flow. The liquids on Titan are therefore composed of hydrocarbons such as methane and ethane, Discovery News reported.
"Though there are some short, local meanders, the relative straightness of the river valley suggests it follows the trace of at least one fault, similar to other large rivers running into the southern margin of this same Titan sea," said Jani Radebaugh, a Cassini radar team associate at Brigham Young University, Utah.
"Such faults - fractures in Titan's bedrock - may not imply plate tectonics, like on Earth, but still lead to the opening of basins and perhaps to the formation of the giant seas themselves," Radebaugh said.
In Titan's equatorial regions, images from Cassini's visible-light cameras in late 2010 revealed regions that darkened due to recent rainfall. Cassini's visual and infrared mapping spectrometer confirmed liquid ethane at a lake in Titan's southern hemisphere known as Ontario Lacus in 2008.
"Titan is the only place we've found besides Earth that has a liquid in continuous movement on its surface," said Steve Wall, the radar deputy team lead, based at NASA's Jet Propulsion Laboratory.
"This picture gives us a snapshot of a world in motion. Rain falls, and rivers move that rain to lakes and seas, where evaporation starts the cycle all over again. On Earth, the liquid is water; on Titan, it's methane; but on both it affects most everything that happens," Wall said.
The radar image taken on September 26, 2012 shows Titan's north polar region, where the river valley flows into Kraken Mare, a sea that is, in terms of size, between the Caspian Sea and the Mediterranean Sea on Earth.