We have obtained 13 moderate-resolution (similar to 2.5 Angstrom) spec
tra of the Galactic X-ray nova GS 2000+25 in quiescence with the W. M.
Keck 10 m telescope. Cross-correlation with the spectra of late-type
dwarfs (especially K2-K7) yields excellent radial velocities for the s
econdary star. The orbital period is consistent with that previously o
btained from photometry (similar to 8.3 hr). A semiamplitude of 518.4
+/- 3.5 km s(-1) is found, similar to 25% smaller than the outer disk
velocity implied by the double-peaked H alpha emission line (Delta v/2
approximate to 700 km s(-1)). The derived mass function is 4.97 +/- 0
.10 Mo, the second highest known for any low-mass X-ray binary. Being
considerably above the conventional limiting mass for a neutron star (
similar to 3.2 M.), it strongly suggests that the compact primary is a
black hole. Plausible mass estimates based on M(2) = 0.4-0.7 M. and i
= 67 degrees 5-80 degrees are M(1) = 5.9-7.5 M.. Under the assumption
that the radial velocity curve of the centroid of the Ha emission lin
e represents the true motion of the black hole, we formally find q = M
(2)/M(1) = 0.050 +/- 0.026.