We describe a micromachined magnetic-field sensor that is based on an electron tunneling transducer. This tunnel sensor is small, very sensitive, operates at ambient temperature and requires very little power. The measured resolution of the sensor is 0.3 nT/√Hz at 1 Hz. The limiting resolution, calculated based on fundamental noise sources, is 0.002 nT/√Hz at 1 Hz. The dominant source of the observed noise in the present device is low frequency air pressure fluctuations.