Titan is the only other body in the solar system besides earth with liquid oceans on its surface.
An initial proposal was to send a boat to explore the seas of Titan.
"If you only explored our ocean with a boat, you would miss out on a heck of a lot of stuff," Steven Oleson from NASA's Glenn Research Centre told science magazine New Scientist.
NASA now plans to send both the boat and submarine to study seas on Titan.
The space submarine, part of NASA's Innovative Advanced Concepts (NIAC), should be able to communicate from beneath the waves.
"The Titan submarine would serve as a pathfinder for even more exotic future subsurface exploration of oceans in space," Oleson added.