Selective targeting of cancer cells is a major challenge for cancer therapy. Many cancer cells overexpress the cystine/glutamate antiporter xCT/CD98, a L-cystine transport system that strengthens antioxidant defenses, thereby promoting tumour survival and progression. Here, we show that the D-enantiomer of cysteine (D-Cys) is selectively imported into xCT/CD98-overexpressing cancer cell lines and impairs their proliferation, particularly under high oxygen concentrations. Intracellular D-Cys specifically inhibits the mitochondrial cysteine desulfurase NFS1, a key enzyme of cellular iron-sulfur protein biogenesis, by blocking sulfur mobilization due to steric constraints. NFS1 inhibition by D-Cys affects all cellular Fe-S cluster-dependent functions, including mitochondrial respiration, nucleotide metabolism, and maintenance of genome integrity, leading to decreased oxygen consumption, DNA damage, and cell cycle arrest. D-Cys administration diminishes tumour growth of human triple-negative breast cancer cells implanted orthotopically into the mouse mammary gland. Hence, D-Cys could represent a simple therapy to selectively target those forms of cancer characterized by overexpression of xCT/CD98.