Planar magnetic sensor, made in particular via CMOS techniques on a semiconductor substrate (1) of for example parallelepiped shape. It includes an amorphous ferromagnetic core (10) in the shape of a Greek cross which occupies the two diagonals of the square defined by the outer contour (90) of the excitation coil (9), the latter being made in the form of a planar winding of square shape. One thus measures, via flat detection coils (70, 80 and 71, 81) which are mounted in series and in a differential arrangement, the two orthogonal components (H1, H2) of the external magnetic field (Hext).