<p>Phenothiazine derivatives have gained increasing attention as potential anticancer agents, while copper-based complexes represent promising cancer therapeutic candidates due to their redox activity, coordination versatility, and biological compatibility. In this study, five novel ternary copper(II) complexes were synthesized by combining phenothiazine derivatives with amino acids as secondary ligands to enhance structural diversity, solubility, and cellular uptake. The aim was to investigate their cytotoxic potency, selectivity, and underlying molecular mechanisms against human cancer cell lines, with a particular focus on hepatocellular carcinoma. The five copper(II) complexes were synthesized by combining phenothiazine derivatives—promazine (Prom) and triflupromazine (TFP)—with biologically relevant amino acids (methionine, tryptophan, tyrosine, and serine) as secondary ligands. Using MTT assays, the synthesized complexes [Cu(Prom)(Meth)Cl], [Cu(Prom)(Tryp)Cl]H₂O, [Cu(Prom)(Tyr)Cl], [Cu(Prom)(Ser)Cl], and [Cu(TFP)(Ser)Cl] were assessed for their in vitro anticancer efficacy against five human cancer cell lines. Among them, [Cu(TFP)(Ser)Cl] exhibited the highest cytotoxic potency and selectivity toward HepG2 liver cancer cells, with an IC₅₀ value of 15.15 µg/mL. Mechanistic studies revealed that this complex induces apoptosis, cell cycle arrest at G₂/M phase, and autophagy, as confirmed by flow cytometric analysis. Gene expression analysis showed significant downregulation of the antiapoptotic BCL2 gene, marked upregulation of the autophagy marker LC3, and moderate upregulation of CDK4. These results were confirmed by molecular docking experiments, which revealed advantageous interactions with proteins involved in cell cycle regulation and apoptosis. These results highlight phenothiazine–copper–amino acid complexes as promising anticancer agents and identify [Cu(TFP)(Ser)Cl] as a potential candidate for hepatocellular carcinoma therapy.</p> Graphical Abstract <p></p>