Astrophysicists pored over information sent back by the orbiter's imaging spectrometer, which is able to detect elements in a planet's surface or atmosphere thanks to the spectrum of light that is reflected from the Sun.
Reporting in Thursday's issue of Nature, the British weekly science journal, the French-led team say they have spotted frozen water in three forms in the Martian south pole.
The first is water ice mixed with "large concentrations" of frozen carbon dioxide (CO2) on a large bright spot on the perennial polar cap -- the cap that is there all year round.
Beyond the boundary of the perennial cap, the frozen area advances and retreats in line with Mars' summer and winter.
Exactly how much of the ice on this bright spot is water rather than CO2 is unclear. A good estimate would be about 15 percent, the scientists say.
The second form is in icy deposits that encrust rugged scarps around the polar cap, and which appear to be free of CO2.
But the most exciting find is huge icy deposits lying some distance away that seem to be a mixture of water and dust.
This ice is present "along vast zones expanding down slope in stratified terrains, tens of kilometers (miles) wide, and tens of kilometers (miles) away" from the bright cap, the study says.
As for the perennial cap, laser altimeter readings suggest it is a slab between one and three kilometres (half a mile to two miles) thick and some 400 kilometres (250 miles) across.
The good news is that the bright spot which is mainly CO2 is only a small part of the cap.
In the areas adjacent to it, the carbon dioxide ice "might well be restricted to a fairly thin layer, no more than some metres (yards) in depth," the authors hope.
If so, the implication is that the rest of this large slab comprises dust and water ice, constituting "a significant fraction" of Mars' water reservoir.
The study is led by Jean-Pierre Bibring, of the Institut d'Astrophysique Spatiale near Paris.
Mars' north pole has long been considered to comprise mainly of water ice, and there were suspicions that frozen water, mixed with CO2, also existed at the south pole, although scientists lacked solid proof until Mars Express sent back its first findings on January 23.
On March 2, NASA said that its robot probe Opportunity had landed in an area that clearly had once been awash with liquid water and which would have been a habitable environment.
The search for water is central to the US and European missions to Mars.
Water nurtures life forms and enables them to evolve.
In the case of Earth, the first forms of life, no doubt bacterial, are believed to have emerged in the oceans that cover two-thirds of the planet's surface.
The presence of frozen water suggests that, when Mars was warmer and had an atmosphere, it too could once have been a haven for life.
Finding liquid water would be an even more exciting find than frozen water, because it is the right temperature at which many elements dissolve in it and react. This does not happen with ice.