We report the discovery of a widely separated (258$farcs3pm0farcs$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.
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