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Narrow graphene nanoribbons (GNRs) and GNR/single-walled carbon nanotube (SWNT) intramolecular heterojunctions are ideal candidates to construct next-generation electronic and optoelectronic devices. However, the fabrication of high-quality long sub-5 nm wide GNRs and GNR/SWNT heterojunctions is a great challenge. Here, we report a method to produce high-quality sub-5 nm wide GNRs with smooth edges and GNR/SWNT intramolecular heterostructures via palladium-catalyzed full and partial unzipping of SWNTs, respectively. The resulting GNRs could be as narrow as 2.2 nm and had an average length of over 1 μm. By adjusting the unzipping time and the deposited positions of palladium nanoparticles, controlled multiple GNR/SWNT heterostructures were also fabricated on an individual parent SWNT. A GNR field-effect transistor (FET) constructed by a 3.1 nm wide GNR could simultaneously achieve a high on/off current ratio of 1.1 × 104 and a large mobility of 598 cm2 V–1 s–1. The photovoltaic device based on a single GNR (2.4 nm in width)/SWNT (0.8 nm in diameter) heterojunction exhibited a large open-circuit voltage (V oc) of 0.52 V and a high external power conversion efficiency (η) of 4.7% under the 1550 nm wavelength illumination of 931 mW cm–2. Our method provides a pathway to controllably prepare high-quality sub-5 nm GNRs and GNR/SWNT heterojunctions for fundamental studies and practical applications in the electronic and optoelectronic fields.