A water purification system comprising a three-dimensional structure consisting of a plurality of plates interconnected so as to form walls so that said structure has at least an open space which coincides with its lower portion, wherein said structure is apt to be placed on the surface of the water in a lake or sea, so that the water surface where said structure is placed contributes to the hermetic sealing of the latter, wherein the system comprises: at least one source of heat, apt to be placed, in use, under the surface of the water, capable of heating the surrounding portion of water and of generating a corresponding amount of water vapour, at least one conduit apt to connect said water vapour with a source of cold air so as to transform said water vapour into purified water; at least one tank to store said purified water; characterised in that the system comprises at least one element supporting said three-dimensional structure, wherein said supporting element is anchored to the bottom of said lake or sea, or is anchored to an adequate supporting surface placed at a given depth of said lake or sea, in that said source of heat is constituted by at least one electrical resistor, in that said heat source is associated with suitable energy supply, and in that the three-dimensional structure consists of a right-angled triangular prism, having a first rectangular face (y1), which is the surface of the water on which said three-dimensional structure is apt to be placed, a second rectangular face (y2), orthogonal to the first rectangular face, a third rectangular face (y3) that joins far-ends of said first rectangular face (y1) and second rectangular face (y2), and two closed lateral faces in the shape of a right-angled triangle, wherein said first rectangular surface (y1) has a width from 6 to 10 meters and a minimum length of 10 meters and said plates are made of material selected from the group comprising plexiglass, glass, photovoltaic panels. at least one source of heat, apt to be placed, in use, under the surface of the water, capable of heating the surrounding portion of water and of generating a corresponding amount of water vapour, at least one conduit apt to connect said water vapour with a source of cold air so as to transform said water vapour into purified water; at least one tank to store said purified water; characterised in that the system comprises at least one element supporting said three-dimensional structure, wherein said supporting element is anchored to the bottom of said lake or sea, or is anchored to an adequate supporting surface placed at a given depth of said lake or sea, in that said source of heat is constituted by at least one electrical resistor, in that said heat source is associated with suitable energy supply, and in that the three-dimensional structure consists of a right-angled triangular prism, having a first rectangular face (y1), which is the surface of the water on which said three-dimensional structure is apt to be placed, a second rectangular face (y2), orthogonal to the first rectangular face, a third rectangular face (y3) that joins far-ends of said first rectangular face (y1) and second rectangular face (y2), and two closed lateral faces in the shape of a right-angled triangle, wherein said first rectangular surface (y1) has a width from 6 to 10 meters and a minimum length of 10 meters and said plates are made of material selected from the group comprising plexiglass, glass, photovoltaic panels. <EOS>