Despite being the earliest reported case of polymorphism in a molecular crystal, the unstable form II of benzamide has thus far evaded thorough structural characterization because collection of experimental data at atomic resolution has proven extremely challenging. Using a highly validated computational crystal structure prediction (CSP) method based on dispersion-corrected density functional theory, we correctly predict the stable form I with the lowest energy among all sampled structures and its polytypic form III with slightly higher energy. From Rietveld refinement of selected CSP models against synchrotron X-ray powder diffraction data of the historical polymorph, we are able to identify a subtle weakness in the available experimental data and arrive at a revised structure of form II. The revised crystal structure is the first benzamide structure to form catemers rather than dimers and possesses the rare space-group symmetry Fdd2 with two molecules in the asymmetric unit, which is necessary to support the new hydrogen bonding network. This rare space group is only found in the CSP by a complete structural search in all 230 space groups with one or two molecules in the asymmetric unit.