This paper presents a novel method for identifying in real time the mass of an off-road vehicle using measurements of sprung and unsprung mass acceleration. The online estimate can be used for vehicle control strategies such as active safety control, traction control, and powertrain control. The online estimate is needed for vehicles whose mass varies significantly from one loading condition to another. Existing off-road mass estimation strategies that use suspension measurements typically require either known suspension force actuation or a priori knowledge of terrain characteristics. Our unique method for estimating online the mass of an offroad vehicle addresses existing limitations by applying baseexcitation concepts to make the measured unsprung mass acceleration become a known input to the recursive leastsquares estimator. We present computer simulations to demonstrate the method, and conclude that the method provides a practical solution for real-time, off-road vehicle mass estimation.