SUN LOUNGER ASSEMBLY, comprising: - a sun lounger having the surface for lying down made from canvas or from other similar material, of the type comprising means for supporting the back of a person and constraint means for constraining said support means to a flat support, the sun lounger providing side members which support the surface of the sun lounger, - and a removable accessory attached to said sun lounger, - so as to obtain, by combining the accessory to the lounger, a seat, as well as means for supporting the back, - the assembly being characterised in that the accessory foresees a backrest, supported by a pair of uprights, arranged laterally, each of the lower ends of the two uprights being hinged to a bar, the two bars being parallel to one another and being able to be arranged above the surface of the lounger, in a position that is transversal with respect to the longitudinal extension direction of the lounger itself, the two bars being connected to one another by means of a cross-piece, both the bars having a hook-shaped element, suitable for hooking onto one of the side members present in the lounger, being foreseen means for locking the uprights in a lifted position so as to allow the stable positioning in such a position (sitting position) when the device is effectively used, being also foreseen that the length of the bars is such that the ends thereof, without the hook-shaped structures, once the accessory has been arranged above the lounger correctly, rest exactly on the side member of the sun lounger opposite that at which there is the hooking between the bars and the opposite side member of the lounger. - a sun lounger having the surface for lying down made from canvas or from other similar material, of the type comprising means for supporting the back of a person and constraint means for constraining said support means to a flat support, the sun lounger providing side members which support the surface of the sun lounger, - and a removable accessory attached to said sun lounger, - so as to obtain, by combining the accessory to the lounger, a seat, as well as means for supporting the back, - the assembly being characterised in that the accessory foresees a backrest, supported by a pair of uprights, arranged laterally, each of the lower ends of the two uprights being hinged to a bar, the two bars being parallel to one another and being able to be arranged above the surface of the lounger, in a position that is transversal with respect to the longitudinal extension direction of the lounger itself, the two bars being connected to one another by means of a cross-piece, both the bars having a hook-shaped element, suitable for hooking onto one of the side members present in the lounger, being foreseen means for locking the uprights in a lifted position so as to allow the stable positioning in such a position (sitting position) when the device is effectively used, being also foreseen that the length of the bars is such that the ends thereof, without the hook-shaped structures, once the accessory has been arranged above the lounger correctly, rest exactly on the side member of the sun lounger opposite that at which there is the hooking between the bars and the opposite side member of the lounger.