We report the detection of the secondary star spectrum in the hard X-ray transient GRO J0422+32 during its lowest quiescent state. From the amplitude of the radial velocity curve we derive a mass function f(M)=0.85+/-0.30M_solar. We classify the companion star as type M2+/-2V which contributes 35+/-6 per cent of the total flux in the spectral range lambdalambda6000-6500 and 52+/-8 per cent in the range lambdalambda6700-7500. Ellipsoidal modulation fits to the I-band light curves constrain the inclination to be between 41 deg+/-6 deg and 43 deg+/-6 deg for mass ratios q(=M_x/M_c)=10-5. We show that the primary is likely to be a low-mass black hole with a probable mass of ~=2.5-5M_solar.