The data showed something strange happening in the moon's interior which indicated the presence of liquid water.
Writing in the journal Science, the planetary experts led by Dr Luciano Iess, from La Sapienza University in Rome, concluded: "Such a large response to the tidal field requires that Titan's interior is deformable over time scales of the orbital period, in a way that is consistent with a global ocean at depth."
Titan is now believed to belong to an elite "club" of moons with sub-surface oceans. Other members include Jupiter's moons Europa, Ganymede and Callisto.
Scientists have speculated about the possibility of life evolving in the hidden oceans. But there are doubts about whether anything could survive on icy Titan.
In a news story accompanying the research paper, Science writer Richard Kerr says: "Inside Europa, planetary scientists believe hot springs on a rocky ocean floor may be belching all the nutrients and inorganic building blocks required by life. But other kinds of gravity studies show that Titan has a cold, icy ocean floor, so no hot water can be leaching life-giving chemicals into the ocean."