New copper(I) complexes of four methyl 4-aryl-6-methyl-3,4-dihydropyrimidine-2(1H)-thione-5-carboxylate ligands, where the aryl group is phenyl (HL1), 1-naphthalenyl (HL2), 2-bromophenyl (HL3) and 2-chlorophenyl (HL4), have been synthesized. Structural characterization was carried out using different techniques, such as elemental analysis, mass spectrometry, molar conductivity, magnetic susceptibility, FTIR, UV–Vis/luminescence and NMR spectroscopies, and allowed us to propose the general formula [CuCl(HL)2] for all complexes. Furthermore, X-ray studies of ligands HL2 and HL3 and the complex [CuCl(HL2)2]·3dmso have been performed. The complex crystallizes in the triclinic space group View the MathML source and adopts a less common, almost perfect, trigonal planar coordination geometry, through the two sulfur atoms of the ligands and one chlorine atom bonded to the metal. The intramolecular hydrogen bonds between the chlorine and the N1H stabilize the structure of the complex. Evaluation of the in vitro activity against human breast cancer representatives of hormonal receptor positive (MCF7) and triple negative (HCC1806) tumors revealed a higher activity in the complexes than in the ligands. Considering particularly HCC1806 cells for which no targeted treatment is available, complexes [CuCl(HL1)2] and [CuCl(HL3)2] exhibited IC50 values of 5.51 and 5.30 μM, respectively, which are lower than the currently used chemotherapeutic drugs.