Many proposed applications of graphene require the ability to tune its electronic structure at the nanoscale(1,2). Although charge transfer(3) and field-effect doping(4) can be applied to manipulate charge carrier concentrations, using them to achieve nanoscale control remains a challenge. An alternative approach is 'self-doping'(5), in which extended defects are introduced into the graphene lattice. The controlled engineering of these defects represents a viable approach to creation and nanoscale control of one-dimensional charge distributions with widths of several atoms(6). However, the only experimentally realized extended defects so far have been the edges of graphene nanoribbons(7-10), which show dangling bonds that make them chemically unstable(11-13). Here, we report the realization of a one-dimensional topological defect in graphene, containing octagonal and pentagonal sp(2)-hybridized carbon rings embedded in a perfect graphene sheet. By doping the surrounding graphene lattice, the defect acts as a quasi-one-dimensional metallic wire. Such wires may form building blocks for atomic-scale, all-carbon electronics.