This paper describes a micromachined magnetic field sensor based on magnetic resonant structures. A micromechanical resonator fabricated using surface micromachining techniques is modified so as to incorporate a magnetic material. The shift of the fundamental mechanical resonant frequency of the device, caused by the interaction of the external magnetic field and the magnetic component of the resonant system, is used to determine the amplitude or the direction of the external field. We have designed, fabricated and tested two types of micromachined magnetic field sensors relying on the proposed principle of operation. The fabrication of the sensors follows CMOS-compatible and low temperature processes based on surface micromachining. Devices have been fabricated which exhibit a minimum resolution of 45° at 30 µT or less, at an excitation voltage of 10 V, demonstrating their utility as a magnetic compass. The power consumed to actuate the resonator is on the order of 20 nW. A theoretical model of the magnetic field sensor was developed using vibration analysis and nonlinear deflection theory. Good agreement was observed between the predicted and observed behavior of the compass.