Stars like our sun fuse hydrogen into helium and in the process produce energy, most of which appears as light. Current stellar models agree that brown dwarfs -- objects less massive than about 80 M_J (where M_J is the mass of Jupiter) -- cannot sustain hydrogen fusion. The structure of brown dwarfs and large giant planets like Jupiter is the same. Young brown dwarfs, like a young Jupiter, derive energy from gravitational contraction and are quite luminous. Later, without any new source of energy, they radiate their meager internal heat. Thus they rapidly cool and fade away from the sky. For this reason, brown dwarfs have been a preferred dark matter candidate. We report the discovery of a cool (900 K) brown dwarf of of mass 40 M_J. The infrared spectrum of the object is very similar to that of Jupiter! We see clear evidence for methane (as in Jupiter and Saturn) and water. Like the gas planets, it appears that all the refractory metals are no longer present in the atmosphere.