The crystal structure of diroximel fumarate has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional theory techniques. Diroximel fumarate crystallizes in space group P-1 (#2) with a = 6.12496(15), b = 8.16516(18), c = 12.7375(6) Å, α = 85.8174(21), β = 81.1434(12), γ = 71.1303(3)°, V = 595.414(23) Å3, and Z = 2 at 298 K. The crystal structure consists of interleaved double layers of hook-shaped molecules parallel to the ab-plane. The side chains form the inner portion of the layers, and the rings comprise the outer surfaces. There are no classical hydrogen bonds in the structure, but 9 C▬H⋯O hydrogen bonds contribute to the crystal energy. The powder pattern has been submitted to ICDD for inclusion in the Powder Diffraction File™ (PDF®).