Drainage system for horizontally sliding doors or windows, wherein said system comprises: - a fixed transverse element provided with at least a rail adapted to support at least one door or window leaf, at least one channel extending parallel with respect to said rail and at least one tubular duct extending along the longitudinal extension of said transverse element and having an outer lateral wall and an upper wall defining the bottom of said channel, wherein said upper wall is crossed by at least a drainage opening and said outer lateral wall is crossed by at least a slot, - a drainage tray insertable into said tubular duct through said slot and adapted to extend under said drainage opening, wherein said drainage tray has a front wall, which is adapted to cover said slot and is provided with at least a discharge opening, and a rear wall, which is opposed with respect to said front wall and is adapted to be housed within said tubular duct, characterised in that - said drainage tray comprising at least an intermediate support obtained in correspondence of at least a respective position defined between said front wall and said rear wall at a not null distance with respect to the same and suitable for rotatably supporting at least a respective door about a rotation axis transverse to said drainage tray between a first position, in which said door is lifted toward said front wall away from said rear wall, and a second position, in which said door is substantially parallel with respect to said front wall thus realizing a septum dividing said drainage tray in respective longitudinal compartments, wherein said at least one drainage opening opens into the downward compartment, with respect to the introduction direction of said drainage tray into said tubular duct, of said septum. - a fixed transverse element provided with at least a rail adapted to support at least one door or window leaf, at least one channel extending parallel with respect to said rail and at least one tubular duct extending along the longitudinal extension of said transverse element and having an outer lateral wall and an upper wall defining the bottom of said channel, wherein said upper wall is crossed by at least a drainage opening and said outer lateral wall is crossed by at least a slot, - a drainage tray insertable into said tubular duct through said slot and adapted to extend under said drainage opening, wherein said drainage tray has a front wall, which is adapted to cover said slot and is provided with at least a discharge opening, and a rear wall, which is opposed with respect to said front wall and is adapted to be housed within said tubular duct, characterised in that - said drainage tray comprising at least an intermediate support obtained in correspondence of at least a respective position defined between said front wall and said rear wall at a not null distance with respect to the same and suitable for rotatably supporting at least a respective door about a rotation axis transverse to said drainage tray between a first position, in which said door is lifted toward said front wall away from said rear wall, and a second position, in which said door is substantially parallel with respect to said front wall thus realizing a septum dividing said drainage tray in respective longitudinal compartments, wherein said at least one drainage opening opens into the downward compartment, with respect to the introduction direction of said drainage tray into said tubular duct, of said septum.