The K-4 structure was theoretically predicted for trivalent chemical species, such as sp(2) carbon. However, since attempts to synthesize the K-4 carbon have not succeeded, this allotrope has been regarded as a crystal form that might not exist in nature. In the present work, we carried out electrochemical crystallization of the radical anion salts of a triangular molecule, naphthalene diimide (NDI)-Delta, using various electrolytes. X-ray crystal analysis of the obtained crystals revealed the K-4 structure, which was formed by the unique intermolecular pi overlap directed toward three directions from the triangular-shape NDI-Delta radical anions: Electron paramagnetic resonance and static magnetic measurements confirmed the radical anion state of NDI-Delta and indicated an antiferromagnetic intermolecular interaction With the Weiss constant of theta = -10 K. The band structure calculation suggested characteristic features of the present material, such as a metallic ground state, Dirac cones, and flat bands.