THE accretion of gas from a companion star onto a compact object, such as a neutron star or black hole, can release large amounts of energy. Episodic accretion is generally thought to explain transient X-ray emission, such as that associated with the recently discovered(1) Galactic superluminal source GRO J1655-40, and to be connected with the ejection of material at relativistic velocities(2). The only other known Galactic superluminal source(2) is difficult to study because it is obscured by intervening gas and dust. Here we report the discovery of the optical counterpart to GRO J1655-40, which we estimate to lie at a distance of only 3 kpc. We have identified the precursor on historical photographic plates; at the time of the X-ray burst in August 1994(1), it brightened by 3 mag in the V bandpass, Although the 12-day delay between the X-ray burst(1) and the main radio outburst which marked the beginning of superluminal motion(3) poses a puzzle that remains to be explained, our results show that at optical wavelengths the characteristics of GRO J1655-40 are similar to those of other accreting-black-hole candidates(4-7).