The application of novel technologies to silicon electronics has been intensively studied with a view to overcoming the physical limitations of Moore's law, that is, the observation that the number of components on integrated chips tends to double every two years. For example, silicon devices have enormous potential for photonic integrated circuits on chips compatible with complementary metal-oxide-semiconductor devices, with various key elements having been demonstrated in the past decade(1-6). In particular, a focus on the exploitation of the Raman effect has added active optical functionality to pure silicon(7-10), culminating in the realization of a continuous-wave all-silicon laser(11). This achievement is an important step towards silicon photonics, but the desired miniaturization to micrometre dimensions and the reduction of the threshold for laser action to microwatt powers have yet to be achieved: such lasers remain limited to centimetre-sized cavities with thresholds higher than 20 milliwatts(12), even with the assistance of reverse-biased p-i-n diodes. Here we demonstrate a continuous-wave Raman silicon laser using a photonic-crystal, high-quality-factor nanocavity without any p-i-n diodes, yielding a device with a cavity size of less than 10 micrometres and an unprecedentedly low lasing threshold of 1 microwatt. Our nanocavity design exploits the principle that the strength of light-matter interactions is proportional to the ratio of quality factor to the cavity volume and allows drastic enhancement of the Raman gain beyond that predicted theoretically(13,14). Such a device may make it possible to construct practical silicon lasers and amplifiers for large-scale integration in photonic circuits.