Since the deposits were scattered around 200 sites in the southern region, it's unlikely that they are the remains of an ocean, scientists reported in Friday's issue of the journal Science.
The salt deposits were found by NASA's orbiting Mars Odyssey, which used its thermal imaging camera to scan the Martian surface.
"Many of the deposits lie in basins with channels leading into them," mission scientist Philip Christensen of Arizona State University said in a statement. "This is the kind of feature, like salt-pan deposits on Earth, that's consistent with water flowing in over a long time."
The Mars Odyssey is managed by NASA's Jet Propulsion Laboratory