We present the discovery of a circumstellar dust disk surrounding AU Microscopii (AU Mic, GJ 803, HD 197481). This young M star at 10 parsec has the same age and origin as β Pictoris, another nearby star surrounded by a dust disk. The AU Mic disk is detected between 50 astronomical units (AU) and 210 AU radius, a region where dust lifetimes exceed the present stellar age. Thus, AU Mic is the nearest star where we directly observe the solid material required for planet formation. Because 85% of stars are M-type, the AU Mic disk provides new clues on how the majority of planetary systems might form and evolve.