Novel solid forms of imatinib mesilate comprising as active ingredient 4-[(4-methyl-1-piperazinyl)methyl]-N-[4-methyl-3-[[4-(3-pyridinyl)-2-pyrimidinyl]amino]-phenyl]-benzamide methanesulfonic acid salt and as co-crystal former benzoic acid, fumaric acid or succinic acid are described. The said multi-component crystalline forms possess improved physical and biological properties with respect to the crystalline forms of the active pharmaceutical ingredient previously known.