To prepare supramolecular stacks of nanographenes, chemists have historically relied heavily on the use of assembly-assisting substituents. We have now discovered the first-in-class polycyclic π-system, consisting solely of sp 2 -hybridized carbon atoms, that spontaneously stacks without any assembly-assisting substituents. Our new, well-designed nanographene with negative curvature self-assembles efficiently with offset π-π stacking (45° twist angles in the present case), ultimately creating beautiful, double-helix, supramolecular nanofibers. Based on this discovery and its revelation of a new guiding principle in supramolecular self-assembly, we expect that a number of negatively curved nanographenes can be developed for new applications in materials science and biology. Moreover, this work not only reports the discovery of an all-sp 2 -carbon supramolecular π-organogelator with negative curvature, but it also showcases the power of 3D electron crystallography for the structural determination of submicrometer-sized hydrocarbon molecular assemblies. This ability to elucidate the layered structures adopted by negatively curved nanographenes at the molecular level will accelerate the future development of carbon-based materials science and technology.