A main biochemical property of cancer cells, compared with normal cells, is altered redox status including increased levels of copper to maintain their malignant phenotypes. Thus, increasing copper accumulation, by using ionophores, to disrupt abnormal redox homeostasis of cancer cells may be an important anticancer strategy. Naturally occurring molecules with extraordinarily diverse chemical scaffolds are an important source of inspiration for developing copper ionophores. Dietary flavonoids are well-characterized copper chelators and show cancer chemopreventive potential, but their ionophoric role for redox-active copper and the related biological implications have remained unknown. This study reports, for the first time, the structural basis, chemical driving forces and biological implications of flavones (a widely distributed subgroup of flavonoids) as Cu(II) ionophores, and also provides new insights into cancer chemopreventive mechanism of flavones bearing 3(or 5)-hydroxy-4-keto group. 3-Hydroxyflavone surfaced as a potent Cu(II) ionophore to induce the mitochondria-dependent apoptosis of cancer cells in a redox intervention fashion via sequential proton-loss Cu(II) chelation, GSH-driving releasing of copper and protonation-dependent efflux of the neutral ligand. Display omitted •Structural basis of flavones as Cu(II) ionophores is 3(or 5)-hydroxy-4-keto group.•3-Hydroxyflavone (3-HF) surfaced as a potent Cu(II) ionophore to kill cancer cells.•The first step of 3-HF as a Cu(II) ionophore is sequential proton-loss chelation.•The second step of 3-HF as a Cu(II) ionophore is GSH-driving copper releasing.•The third step of 3-HF as a Cu(II) ionophore is protonation-dependent efflux.