A MEMS inertial sensor comprising: a supporting structure (41, 35; 61, 61A; 335); an inertial structure (33, 34; 134, 234; 333, 433, 334, 434) having a main extension in a plane; and a first elastic structure (37; 37A, 37B; 337, 437), elastically coupling the inertial structure to the supporting structure and configured to enable a movement of the inertial structure along a sensing axis lying in the plane as a function of a quantity to be detected (a ext); wherein the inertial structure comprises: a first inertial mass (33; 333, 433) of semiconductor material; a second inertial mass (34; 134, 234; 334, 434) of semiconductor material; a second elastic structure (38; 138, 238; 338, 438), elastically coupling the first and second inertial masses and configured to enable a movement of the second inertial mass along the sensing axis; and a fixed stop structure (50, 51; 65; 350, 351, 450, 451) facing the first inertial mass, at least one electrode (43, 45; 343, 345, 443, 445) capacitively coupled to the first inertial mass (33; 333, 433) and forming therewith at least one capacitor (C 1, C 2) having a capacitance that is a function of the quantity to be detected (a ext), wherein the first elastic structure has a first elastic constant (k 1) and the second elastic structure (38; 138, 238; 338, 438) has a second elastic constant (k 2), the first elastic constant being lower than the second elastic constant so that, in presence of the quantity to be detected, the inertial structure is configured to move in a sensing direction until the first inertial mass stops against the stop structure and the second inertial mass is configured to be able to move further in the sensing direction and, once the quantity to be detected ends, the second inertial mass is configured to move in an opposite direction to the sensing direction and to detach the first inertial mass from the stop structure. a supporting structure (41, 35; 61, 61A; 335); an inertial structure (33, 34; 134, 234; 333, 433, 334, 434) having a main extension in a plane; and a first elastic structure (37; 37A, 37B; 337, 437), elastically coupling the inertial structure to the supporting structure and configured to enable a movement of the inertial structure along a sensing axis lying in the plane as a function of a quantity to be detected (a ext); wherein the inertial structure comprises: a first inertial mass (33; 333, 433) of semiconductor material; a second inertial mass (34; 134, 234; 334, 434) of semiconductor material; a second elastic structure (38; 138, 238; 338, 438), elastically coupling the first and second inertial masses and configured to enable a movement of the second inertial mass along the sensing axis; and a fixed stop structure (50, 51; 65; 350, 351, 450, 451) facing the first inertial mass, at least one electrode (43, 45; 343, 345, 443, 445) capacitively coupled to the first inertial mass (33; 333, 433) and forming therewith at least one capacitor (C 1, C 2) having a capacitance that is a function of the quantity to be detected (a ext), wherein the first elastic structure has a first elastic constant (k 1) and the second elastic structure (38; 138, 238; 338, 438) has a second elastic constant (k 2), the first elastic constant being lower than the second elastic constant so that, in presence of the quantity to be detected, the inertial structure is configured to move in a sensing direction until the first inertial mass stops against the stop structure and the second inertial mass is configured to be able to move further in the sensing direction and, once the quantity to be detected ends, the second inertial mass is configured to move in an opposite direction to the sensing direction and to detach the first inertial mass from the stop structure. a first inertial mass (33; 333, 433) of semiconductor material; a second inertial mass (34; 134, 234; 334, 434) of semiconductor material; a second elastic structure (38; 138, 238; 338, 438), elastically coupling the first and second inertial masses and configured to enable a movement of the second inertial mass along the sensing axis; and a fixed stop structure (50, 51; 65; 350, 351, 450, 451) facing the first inertial mass, at least one electrode (43, 45; 343, 345, 443, 445) capacitively coupled to the first inertial mass (33; 333, 433) and forming therewith at least one capacitor (C 1, C 2) having a capacitance that is a function of the quantity to be detected (a ext), wherein the first elastic structure has a first elastic constant (k 1) and the second elastic structure (38; 138, 238; 338, 438) has a second elastic constant (k 2), the first elastic constant being lower than the second elastic constant so that, in presence of the quantity to be detected, the inertial structure is configured to move in a sensing direction until the first inertial mass stops against the stop structure and the second inertial mass is configured to be able to move further in the sensing direction and, once the quantity to be detected ends, the second inertial mass is configured to move in an opposite direction to the sensing direction and to detach the first inertial mass from the stop structure.