Described herein is an inertial sensor (1) provided with a detection structure (9, 19) sensitive to a first, a second and a third component of acceleration (a x , a y , a z ) along respective directions of detection (x, y, z), and generating respective electrical quantities as a function of said components of acceleration. The detection structure (9, 19) supplies at output a resultant electrical quantity (C) obtained as combination of said electrical quantities, and correlated to the value of a resultant acceleration (a) acting on the inertial sensor (1), given by a vector sum of the components of acceleration (a x , a y , a z ) . In particular, the detection structure (9, 19) is of a microelectromechanical type, and comprises a mobile portion (2, 12) made of semiconductor material forming with a fixed portion (8, 18) a first, a second and a third detection capacitor, and an electrical-interconnection portion (10, 20), connecting the detection capacitors in parallel; the resultant electrical quantity (C) being the capacitance obtained from said connection in parallel.