It is the first river system this vast and in such high resolution seen anywhere beyond Earth, a release from the Paris headquarters of the European Space Agency said Wednesday.
Astronomers say they believe the river, captured in radar images by the orbiting Cassini spacecraft, is filled with liquid because it appears dark along its entire length in the high-resolution image, suggesting a smooth surface.
The river contains not water but liquid ethane and methane, they said.
"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 scientist at Brigham Young University.
Titan is the only other world in the solar system with stable liquid on its surface, the researchers said.