The design, fabrication, and performance of a nanoscopic magnetic field sensor based on the newly discovered phenomenon of extraordinary magnetoresistance (EMR) are reported. It is shown that a sensor with an active volume of 35 nm length×30 nm width×20 nm35 nm length×30 nm width×20 nm height yields room temperature EMR values as high as 35% at an applied field of 0.05 T. The mesoscopic physics implications of these new results are discussed.