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Two novel benzoylthioureas derived from gallic acid, (tri‐O‐acetyl)galloyl‐N,N‐diethylthiourea HL1, and cinnamic acid, cinnamoyl‐N,N‐diethylthiourea HL2 have been successfully prepared and characterized by means of elemental analysis, IR, NMR, high‐resolution MS, and X‐ray crystallography. The organic ligands react with Ni(AcO)2 and Cu(AcO)2 in MeOH under formation of bis‐complexes with the compositions of [M(L)2] (M=Ni2+, Cu2+; L=L1, L2). Similar reactions with Co(AcO)2, however, result in Co(III) tris‐complexes [Co(L)3] (L=L1, L2) with the metal ions oxidized presumably by atmospheric dioxygen. X‐ray crystallography and spectroscopic characterization reveal a cis square‐planar coordination in the bis‐complexes and facial octahedral geometry in the tris‐complexes. In all of metal complexes, the deprotonated organic compounds ({L1}− and {L2}−) serve as (S,O)‐bidentate ligands. The ligand HL1 and its Ni(II) and Cu(II) complexes exhibit weak antiproliferative effects on the human MCF7 breast and HepG2 liver cancer cells with IC50 values in the range of 60–115 μM. Surprisingly, the tris‐complex [Co(L1)3] exhibits high cytotoxicity with IC50 values of 22.23±1.58 μM for MCF7 and 28.30±3.09 μM for HepG2 cancer cells. The activity against MCF7 cells is even more than that of cisplatin under the same conditions. [ABSTRACT FROM AUTHOR]