The ability to tune the properties of graphene nanoribbons (GNRs) through modification of the nanoribbon's width and edge structure(1-3) widens the potential applications of graphene in electronic devices(4-6). Although assembly of GNRs has been recently possible, current methods suffer from limited control of their atomic structure(7-13), or require the careful organization of precursors on atomically flat surfaces under ultra-high vacuum conditions(14). Here we demonstrate that a GNR can self-assemble from a random mixture of molecular precursors within a single-walled carbon nanotube, which ensures propagation of the nanoribbon in one dimension and determines its width. The sulphur-terminated dangling bonds of the GNR make these otherwise unstable nanoribbons thermodynamically viable over other forms of carbon. Electron microscopy reveals elliptical distortion of the nanotube, as well as helical twist and screw-like motion of the nanoribbon. These effects suggest novel ways of controlling the properties of these nanomaterials, such as the electronic band gap and the concentration of charge carriers.