It i1s well known that low-frequency (0.01–10 Hz), 10–100 nanotesla (nT) magnetic fluctuations occur hours or days before an earthquake. This article reports the development of ultrasensitive, low-cost, room-temperature, magnetic sensors that may enable the deployment of such sensors and sensor networks for recording these magnetic precursors to earthquakes. The sensors are based on magnetoelectric (ME) composites that produce a voltage response to the magnetic field fluctuations. The composites have two components: a ferromagnetic layer that responds to the magnetic field by producing a mechanical strain and a piezoelectric layer that converts the mechanical strain to a voltage. Several ferromagnetic-piezoelectric composites were prepared and characterized, and we found the best response in terms of low noise and sensitivity from sensors constructed of Metglas lead zirconate titanate (PZT). The data indicate that the samples can measure 0.1 nT at 0.1–10 Hz, a sensitivity ideal for the detection of 10–100 nT field fluctuations that precede an earthquake. Suggested follow-up efforts include a large-scale deployment of the sensors and sensor networks in earthquake-prone regions for field tests and ultimately the development of a sensor-based early warning system.