  We have obtained a good quality R$\sim$400 0.8-2.5 $\mu$m spectrum and
accurate photometry of Gl 570D, one of the coolest and least luminous brown
dwarfs currently known. The spectrum shows that Gl 570D has deeper absorptions
in the strong water and methane bands at 1.12-1.17 $\mu$m, 1.33-1.45 $\mu$m,
1.62-1.88 $\mu$m, and 2.20-2.45 $\mu$m and is both bluer at J-K and redder at
$K-L^{\prime}$ than previously observed T dwarfs. Data analysis using model
spectra coupled with knowledge of the well-understood primary implies that for
the same surface gravity, Gl 570D is about 160 K cooler than Gl 229B. For an
age range of 2-5 Gyr Gl 570D has an effective temperature in the range 784-824
K, a log gravity in the range 5.00-5.27 cm s$^{-2}$, and a luminosity in the
range 2.88-2.98 $\times$ 10$^{-6}$ L$_\odot$.
