Summary We have developed a synthetic approach to graphene nanoribbons by using butadiyne-containing monomers that are initially converted to polydiacetylenes via topochemical polymerization in the crystal. Subsequent aromatization of the isolated polydiacetylenes at surprisingly mild temperatures affords graphene nanoribbons ∼1.36 nm in width with a 1.4 eV bandgap. These transformations take place solely in the solid state, in contrast to published on-surface or in-solution methods. This synthetic approach is well suited for electronic-device fabrication processes because it requires only UV light or heating and no external chemical reagents.