One-dimensionally connected organic nanostructures with dissimilar semiconducting properties are expected to provide a reliable platform in understanding the behaviors of photocarriers, which are important for the development of efficient photon-to-electrical energy conversion systems. Although bottom-up supramolecular approaches are considered promising for the realization of such nanoscale heterojunctions, the dynamic nature of molecular assembly is problematic. We report a semiconducting nanoscale organic heterojunction, demonstrated by stepwise nanotubular coassembly of two strategically designed molecular graphenes. The dissimilar nanotubular segments, thus connected noncovalently, were electronically communicable with one another over the heterojunction interface and displayed characteristic excitation energy transfer and charge transport properties not present in a mixture of the corresponding homotropically assembled nanotubes.