We report the spectroscopic detection of the secondary star in the x-ray transient QZ Vul (=GS 2000+25). Cross-correlation of individual spectra against K-type templates yields clear velocity modulations with the 8.3 hr orbital period. From the amplitude of the radial velocity curve (520 +/- 16 km/s) we derive a mass function f(M)= 5.02 +/- 0.46 solar mass, the second largest after V404 Cygni. We classify the companion star as a K5+/-2 V which is the dominant light source in our spectral range (lambda lambda 6000-8800). Doppler imaging of the broad H-alpha emission shows evidence for an intense hot spot from which we estimate an accretion disc radius of 0.47 +/- 0.03 RLx.