Device for fastening and levelling sanitary fittings onto walls, comprising: - a laminar body, which in turn comprises: ∘ a front side (1.1) to be arranged facing the sanitary fittings fastened to the laminar body; and ∘ a rear side (1.2) to be arranged facing the wall; - two profiles integrated in the laminar body, one on an upper edge and another one on a lower edge of the laminar body according to the position of the device being fastened to the wall, both profiles being arranged in a displaced plane with respect to the laminar body towards the rear side (1.2); - fastening orifices (2, 3) to fasten the device to the wall, comprising: ∘ a central orifice arranged in a middle point of the longitudinal extension of the profile of the upper edge, and ∘ lateral orifices at least in the profile of the upper and lower edge; - levelling means (5, 6) arranged in the laminar body and each comprising a threaded body and an extension element externally threaded to be threadable through the threaded body, the device being characterized in that the extension element is immobilized by a locknut (6.3) coupled to it, the extension element, further comprising: - a support (6.1) at one end to avoid damaging and provide support against the wall, and - a head (6.2) at the other end serving as handling point of the extension element to be threaded by the user to a greater or lesser extent the head being arranged in the front side (1.1) such that the device is to be fastened and levelled with respect to the three coordinate axes (X, Y, Z) by means of the combined adjustment of the fastening orifices (2, 3) and the levelling means (5, 6), the fastening orifices (2, 3) and the levelling means (5, 6) being handled independently from each other, the extension of each extension element being individually adjustable with respect to the rear side (1.2) and providing support with direct contact against the wall. - a laminar body, which in turn comprises: ∘ a front side (1.1) to be arranged facing the sanitary fittings fastened to the laminar body; and ∘ a rear side (1.2) to be arranged facing the wall; ∘ a front side (1.1) to be arranged facing the sanitary fittings fastened to the laminar body; and ∘ a rear side (1.2) to be arranged facing the wall; - two profiles integrated in the laminar body, one on an upper edge and another one on a lower edge of the laminar body according to the position of the device being fastened to the wall, both profiles being arranged in a displaced plane with respect to the laminar body towards the rear side (1.2); - fastening orifices (2, 3) to fasten the device to the wall, comprising: ∘ a central orifice arranged in a middle point of the longitudinal extension of the profile of the upper edge, and ∘ lateral orifices at least in the profile of the upper and lower edge; ∘ a central orifice arranged in a middle point of the longitudinal extension of the profile of the upper edge, and ∘ lateral orifices at least in the profile of the upper and lower edge; - levelling means (5, 6) arranged in the laminar body and each comprising a threaded body and an extension element externally threaded to be threadable through the threaded body, - a support (6.1) at one end to avoid damaging and provide support against the wall, and - a head (6.2) at the other end serving as handling point of the extension element to be threaded by the user to a greater or lesser extent the head being arranged in the front side (1.1)