Using the techniques of microelectromechanical systems (MEMS), we have constructed a small, low-power magnetic sensor based on alkali atoms.   We use a coherent population trapping resonance to probe the interaction of the atoms’ magnetic moment with a magnetic field and detect changes in the magnetic flux density with a sensitivity of 50 pT/Hz1/2 at 10 Hz.  The magnetic sensor has a size of 12 mm3 and dissipates 195 mW of power.  Further improvements in size, power dissipation, and magnetic field sensitivity are immediately foreseeable, and such a device could provide a hand-held, battery-operated magnetometer with an atom shot-noise-limited sensitivity of 0.05 pT Hz-1/2.