The crystal and molecular structure of the title compound has been determined by X-ray analysis. The 1-(p-fluorophenylmethyl)-2-(4-methyl-1-piperazinyl)-1H-benzimidazole ion cocrystallizes with fumaric acid, (C19H22N4F+. C4H3O4-), in space group P-1 with cell dimensions a = 9.938(1), b = 10.131(1), c = 12.712(1) Angstrom, alpha = 86.57(1), beta = 69.41(2), and gamma = 67.22(2)degrees. The piperazine N4 atom is protonated and contacts the deprotonated O atom of the fumarate anion through a hydrogen bond. The benzimidazole ring is nearly planar and makes a dihedral angle of 111.25(12)degrees with the fluorophenyl ring. The piperazine ring adopts a chair conformation.