An inertial sensor provided with a detection structure sensitive to a first, a second and a third component of acceleration along respective directions of detection, and generating respective electrical quantities as a function of said components of acceleration. The detection structure supplies at output a resultant electrical quantity obtained as combination of said electrical quantities, and correlated to the value of a resultant acceleration acting on the inertial sensor, given by a vector sum of the components of acceleration. In particular, the detection structure is of a microelectromechanical type, and comprises a mobile portion made of semiconductor material forming with a fixed portion a first, a second and a third detection capacitor, and an electrical-interconnection portion, connecting the detection capacitors in parallel; the resultant electrical quantity being the capacitance obtained from said connection in parallel.