The very recent detection of the faint host galaxy of one gamma-ray burst(1-4), and the determination of a cosmological redshift for another(5), demonstrates that these events are the most luminous phenomena in the Universe, emitting more energy in radiation than a supernova over just a few seconds. The source of this energy is still unknown, but may become clear through studies of the counterparts at longer wavelengths. Here we report the detection of an optical counterpart to a gamma-ray burst (GRB971214) that occurred on 14 December 1997, It faded rapidly over a two-week period. just like the previous two optical transients(1,6-11); which dispels any doubt that the three events are the optical afterglows of gamma-ray bursts. The 14 December optical transient is the faintest of the three, and also is much redder than the other two. This reddening probably arises because of scattering by interstellar dust along the line of sight, which is presumably present in the denser regions of the host galaxy, where stars form. This suggests that the burst＇s progenitor did not stray too far from the point of its birth, which, regardless of the nature of the source, appears to be in a region of dense gas.