Persistent efforts have been made to achieve efficient light emission from silicon(1-7) in the hope of extending the reach of silicon technology into fully integrated optoelectronic circuits, meeting the needs for high-bandwidth intrachip and interchip connects(8). Enhanced light emission from silicon is known to be theoretically possible(9,10), enabled mostly through quantum-confinement effects(2-4). Furthermore, Raman-laser conversion was demonstrated in silicon waveguides(11,12). Here we report on optical gain and stimulated emission in uniaxially nanopatterned silicon-on-insulator using a nanopore array as an etching mask(13). In edge-emission measurements, we observed threshold behaviour, optical gain, longitudinal cavity modes and linewidth narrowing, along with a collimated far-field pattern, all indicative of amplification and stimulated emission(14-17). The sub-bandgap 1,278 nm emission peak is attributed to A-centre mediated phononless direct recombination between trapped electrons and free holes(18-20). The controlled nanoscale silicon engineering, combined with the low material loss in this sub-bandgap spectral range and the long electron lifetime in such A-type trapping centres, gives rise to the measured optical gain and stimulated emission and provides a new pathway to enhance light emission from silicon.