We have detected five objects toward a Scutum massive star cluster of red supergiants in the SiO J = 1-0, v = 1 or v = 2 transitions. The radial velocity data indicate that four of the detections are cluster members and that the other, which is located close to the X-ray source AX 1838-0655, is a foreground object. The high velocity resolution of the maser lines provides a more accurate determination of the radial velocity (120 km s-1) and velocity dispersion (~2 km s-1) of the cluster and, hence, of the distance (6.5 kpc) and luminosities of the stars. We discuss the implications of these measurements in constraining the relationships between SiO masers and mass-loss rate and modeling the cluster age and mass.