  We measured the radial velocity curve of the companion of the neutron star
X-ray transient XTE J2123-058. Its semi-amplitude (K_2) of 298.5 +/- 6.9 km/s
is the highest value that has been measured for any neutron star LMXB. The high
value for K_2 is, in part, due to the high binary inclination of the system but
may also indicate a high neutron star mass. The mass function (f_2) of 0.684
+/- 0.047 solar masses, along with our constraints on the companion's spectral
type (K5V-K9V) and previous constraints on the inclination, gives a likely
range of neutron star masses from 1.2 to 1.8 solar masses. We also derive a
source distance of 8.5 +/- 2.5 kpc, indicating that XTE J2123-058 is unusually
far, 5.0 +/- 1.5 kpc, from the Galactic plane. Our measurement of the systemic
radial velocity is -94.5 +/- 5.5 km/s, which is significantly different from
what would be observed if this object corotates with the disk of the Galaxy.
