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Accumulating evidence suggests an association between iron metabolism and lung cancer progression. In biological systems, iron is present in either reduced (Fe2+; ferrous) or oxidized (Fe3+; ferric) states. However, ferrous and ferric iron exhibit distinct chemical and biological properties, the role of ferrous and ferric iron in lung cancer cell growth has not been clearly distinguished. In this study, we manipulated the balance between cellular ferrous and ferric iron status by inducing gene mutations involving the FBXL5–IRP2 axis, a ubiquitin‐dependent regulatory system for cellular iron homeostasis, and determined its effects on lung cancer cell growth. FBXL5 depletion (ferrous iron accumulation) was found to suppress lung cancer cell growth, whereas IRP2 depletion (ferric iron accumulation) did not suppress such growth, suggesting that ferrous iron but not ferric iron plays a suppressive role in cell growth. Mechanistically, the depletion of FBXL5 impaired the degradation of the cyclin‐dependent kinase inhibitor, p27, resulting in a delay in the cell cycle at the G1/S phase. FBXL5 depletion in lung cancer cells also improved the survival of tumor‐bearing mice. Overall, this study highlights the important function of ferrous iron in cell cycle progression and lung cancer cell growth. [ABSTRACT FROM AUTHOR]