  We report the discovery of a widely separated (258$\farcs3\pm0\farcs$4) T
dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was
initially identified from the Two Micron All Sky Survey (2MASS). Its
near-infrared spectrum shows the 1.6 and 2.2 $\micron$ CH$_4$ absorption bands
characteristic of T dwarfs, while its common proper motion with the Gl 570ABC
system confirms companionship. Gl 570D (M$_J$ = 16.47$\pm$0.07) is nearly a
full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and
estimates of L = (2.8$\pm$0.3)x10$^{-6}$ L$_{\sun}$ and T$_{eff}$ = 750$\pm$50
K make it significantly cooler and less luminous than any other known brown
dwarf companion. Using evolutionary models by Burrows et al. and an adopted age
of 2-10 Gyr, we derive a mass estimate of 50$\pm$20 M$_{Jup}$ for this object.
