Inertial sensor able to measure information relating to rotation in three (X, Y, Z) orthogonal axes; said inertial sensor comprising a support (4, 4') and a sensitive element secured to the support (4, 4'); said sensitive element being adapted to vibrate relative to the support (4, 4') in a primary vibration mode when it is excited by an excitation device, and in secondary vibration modes induced by a Coriolis force, when the inertial sensor is moved about an axis and the sensitive element is excited by an excitation device; said sensitive element comprising a deformable frame which extends in a plane (X-Y) and at least two deformable projections (13, 14, 15, 16) which extend in the same plane (X-Y); said primary vibration mode implementing a deformation of the deformable frame; said secondary vibration modes implementing in-plane or out-of-plane bending deformations of said at least two deformable projections (13, 14, 15, 16); characterized in that: ▪ each of said at least two deformable projections (13, 14, 15, 16) is a beam of a substantially square cross-section whose longitudinal axis is rectilinear; ▪ each of said beams (13, 14, 15, 16) being connected only by one of its ends to the deformable frame at a location at which the amplitude of the primary vibration mode is at a maximum so that in the primary vibration mode, each of said beams (13, 14, 15, 16) is translated on its longitudinal axis; ▪ the inertial sensor extends in one and the same plane; ▪ the deformable frame and said at least two deformable projections (13, 14, 15, 16) possess a plane of symmetry parallel to the (X-Y) plane, due to which the translation of said at least two deformable projections (13, 14, 15, 16) occurs without deformation or stress; ▪ the longitudinal axes of said at least two deformable projections (13, 14, 15, 16), along which the latter are displaced by the primary vibration, are non-collinear and are preferably substantially orthogonal; ▪ it further comprises a device for detecting each of the secondary vibration modes; said detection device exploiting an effect caused by the deformation of said beams (13, 14, 15, 16), such as for example magnetostrictive, piezoelectric, piezoresistive, and photoelectric effects. ▪ each of said at least two deformable projections (13, 14, 15, 16) is a beam of a substantially square cross-section whose longitudinal axis is rectilinear; ▪ each of said beams (13, 14, 15, 16) being connected only by one of its ends to the deformable frame at a location at which the amplitude of the primary vibration mode is at a maximum so that in the primary vibration mode, each of said beams (13, 14, 15, 16) is translated on its longitudinal axis; ▪ the inertial sensor extends in one and the same plane; ▪ the deformable frame and said at least two deformable projections (13, 14, 15, 16) possess a plane of symmetry parallel to the (X-Y) plane, due to which the translation of said at least two deformable projections (13, 14, 15, 16) occurs without deformation or stress; ▪ the longitudinal axes of said at least two deformable projections (13, 14, 15, 16), along which the latter are displaced by the primary vibration, are non-collinear and are preferably substantially orthogonal; ▪ it further comprises a device for detecting each of the secondary vibration modes; said detection device exploiting an effect caused by the deformation of said beams (13, 14, 15, 16), such as for example magnetostrictive, piezoelectric, piezoresistive, and photoelectric effects.