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Cr(VI) is broadly applied in industry. Cr(VI) exposure places a big burden on public health, thereby increasing the risk of lung squamous cell carcinoma (LUSC). The mechanisms underlying Cr(VI)‐induced LUSC remain largely elusive. Here, we report that the cancer stem cell (CSC)/tumour‐initiating cell (TIC)‐like subgroup within Cr(VI)‐transformed bronchial epithelial cells (CrT) promotes lung cancer tumourigenesis. Mechanistically, Cr(VI) exposure specifically increases the expression levels of aldehyde dehydrogenase 1A1 (ALDH1A1), a CSC marker, through KLF4‐mediated transcription. ALDH1A1 maintains self‐renewal of CrT/TICs and facilitates the expression and secretion of EGF from CrT/TICs, which subsequently promotes the activation of EGFR signalling in differentiated cancer cells and tumour growth of LUSC. In addition, the ALDH1A1 inhibitor A37 and gemcitabine synergistically suppress LUSC progression. Importantly, high ALDH1A1 expression levels are positively correlated with advanced clinical stages and predict poor survival in LUSC patients. These findings elucidate how ALDH1A1 modulates EGF secretion from TICs to facilitate LUSC tumourigenesis, highlighting new therapeutic strategies for malignant lung cancers. [ABSTRACT FROM AUTHOR]