The development of a robust light source that emits one photon at a time will allow new technologies such as secure communication through quantum cryptography(1). Devices based on fluorescent dye molecules(2), quantum dots(3) and carbon nanotubes(4) have been demonstrated, but none has combined a high single-photon flux with stable, room-temperature operation. Luminescent centres in diamond(5-9) have recently emerged as a stable alternative, and, in the case of nitrogen-vacancy centres, offer spin quantum bits with optical readout(10-15). However, these luminescent centres in bulk diamond crystals have the disadvantage of low photon out-coupling. Here, we demonstrate a single-photon source composed of a nitrogen-vacancy centre in a diamond nanowire, which produces ten times greater flux than bulk diamond devices, while using ten times less power. This result enables a new class of devices for photonic and quantum information processing based on nanostructured diamond, and could have a broader impact in nanoelectromechanical systems, sensing and scanning probe microscopy.