Metal nanocrystals are commonly used to mediate important chemical reactions such as water splitting and CO oxidation. To investigate such redox reactions in detail, it would be useful to be able to carry out electrochemistry on single metal nanocrystals. We report here that the surface plasmon resonance of a single gold nanocrystal can be reversibly and rapidly tuned by tens of nanometers electrochemically. The spectral shifts are more sensitive for elongated morphologies such as rods and lead to color changes perceptible by eye.