This paper reports on a highly sensitive resonant magnetic microsensor with a shift of the mechanical resonance frequency fres depending quadratically on the magnetic flux density B to be measured. The sensor combines an electrostatically driven micromechanical resonator with a geometrically optimized planar magnetic concentrator with two narrow gaps. Sensitivities dfres/dB of several megahertz per tesla are achieved without complex feedback and modulation electronics. When the device is operated in a switched auxiliary magnetic field using two integrated planar coils, the raw parabolic response of the sensor is translated into a resonance frequency difference proportional to B . For a coil current of plusmn120 mA, the device offers a sensitivity of 1.91 MHz/T and a resolution of 1.3 muT . In vacuum, a resolution of 400 nT is demonstrated. With these performance numbers, applications such as a microelectromechanical (MEM) compass for the measurement of the Earth's magnetic field can be envisaged. The fabrication process of the microsensor combines the following: 1) the micromachining of a silicon-on-insulator substrate for the MEM structure; 2) the attachment and structuring of a thin amorphous soft magnetic ribbon for the magnetic concentrator; and 3) the UV-laser ablation for the micropatterning of the concentrator gaps. Thermal effects due to high coil currents were analyzed in detail, as they play an important role in the reported microsensor design.