A sliding pendulum seismic isolator (1) comprises a lower sliding element (2) provided with a concave surface (2a) facing upwards, an upper sliding element (3) provided with a concave surface (3a) facing downwards, a first intermediate element (4) and a second intermediate element (5), said intermediate elements (4, 5) being each provided with a convex sliding surface (4a, 5a) suitable to allow the sliding of the first and second intermediate elements (4, 5) on the concave surfaces (2a, 3a) of said lower and upper sliding elements (2, 3) respectively. The first intermediate element (4) has a convex spherical surface (4b) opposed to its convex sliding surface (4a), and the second intermediate element (5) has a flat surface (5b) opposed to its convex sliding surface (5a). The convex spherical surface (4b) and the flat surface (5b) are in contact with each other and are suitable to allow a relative rotation substantially without sliding between the intermediate elements (4, 5). The isolator according to the present invention shows a remarkable reduction of the parasitic moment against the rotation, thus improving the dynamic response of the isolating system and reducing the stresses inside the materials and the adjacent structures.