  We report the discovery of the coolest field dwarf yet known, selected as a
stellar object with extremely red colors from commissioning imaging data of the
Sloan Digital Sky Survey. Its spectrum from 0.8 to 2.5 microns is dominated by
strong bands of H_2 O and CH_4. Its spectrum and colors over this range are
very similar to those of Gliese 229B, the only other known example of a methane
dwarf. It is roughly 1.2 mag fainter than Gliese 229B, implying that it lies at
a distance of roughly 10 pc. Such a cool object must have a mass well below the
hydrogen-burning limit of 0.08 solar masses, and therefore is a genuine brown
dwarf, with a probable mass in the range 0.015-0.06 solar masses for an age
range of 0.3-5 Gyr.
