Laevens et al. recently discovered Triangulum II, a satellite of the Milky Way. Its Galactocentric distance is 36 kpc, and its luminosity is only 450 L_sun. Using Keck/DEIMOS, we measured the radial velocities of six member stars within 1.2 of the center of Triangulum II, and we found a velocity dispersion of sigma_v = 5.1 -1.4 +4.0 km/s. We also measured the metallicities of three stars and found a range of 0.8 dex in [Fe/H]. The velocity and metallicity dispersions identify Triangulum II as a dark matter-dominated galaxy. The galaxy is moving very quickly toward the Galactic center (v_GSR = -262 km/s). Although it might be in the process of being tidally disrupted as it approaches pericenter, there is no strong evidence for disruption in our data set. The ellipticity is low, and the mean velocity, <v_helio> = -382.1 +/- 2.9 km/s, rules out an association with the Triangulum-Andromeda substructure or the Pan-Andromeda Archaeological Survey (PAndAS) stellar stream. If Triangulum II is in dynamical equilibrium, then it would have a mass-to-light ratio of 3600 -2100 +3500 M_sun/L_sun, the highest of any non-disrupting galaxy (those for which dynamical mass estimates are reliable). The density within the 3-D half-light radius would be 4.8 -3.5 +8.1 M_sun/pc^3, even higher than Segue 1. Hence, Triangulum II is an excellent candidate for the indirect detection of dark matter annihilation.
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