Transcription factors are key regulators of gene expression and play essential roles in various diseases, yet their lack of small-molecule binding pockets makes them difficult to target with drugs. Internal RNA modifications, such as N6-methyladenosine, are widespread in mammalian cells, but how m6A-encoded epitranscriptomic information influences transcription factor activity and cellular function remains incompletely understood. Here, we demonstrate that m6A methylation directly regulates HNF1B and is essential for maintaining redox homeostasis in cancer cells. The METTL3/METTL14 methyltransferase complex deposits m6A marks within the 3′-untranslated regions of HNF1B messenger RNA (mRNA), stabilizing its expression and function. Genetic or chemical inhibition of METTL3-mediated m6A modifications disrupts HNF1B-driven glutathione metabolism, severely impairing the antioxidant capacity of cancer cells and rendering them vulnerable to oxidative stress. Notably, HNF1B loss induces oxidative stress-induced cell death across multiple cancer lineages, mirroring the metabolic dysfunction caused by m6A depletion and establishing HNF1B as a central regulator of redox defense in human cancer. By directly linking RNA modifications of a transcription factor to redox homeostasis, our findings identify the METTL3-HNF1B axis as a metabolic vulnerability in cancer and highlight its potential as a target for m6A-directed cancer therapies.