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Notch receptor activation requires γ-secretase-mediated release of Notch intracellular domain 1(NICD1) to regulate gene transcription. Here, we identify the proton-driven Solute carrier 11A2 (Slc11A2) or divalent metal transport protein Dmt1 as an inhibitor of Notch signaling via regulating iron homeostasis and lysosomal integrity. Dmt1 loss reduces ferritin levels and increases labile Fe²⁺, causing elevated reactive oxygen species (ROS) and lipid peroxidation. These changes compromise lysosomal function and impair degradation of S3-Val1744 cleaved NICD1, resulting in its accumulation. Dmt1 has isoforms with or without an iron response element (IRE): Re-expressing Dmt1+IRE robustly increases ferritin heavy-chain (FTH), whereas Dmt1-IRE moderately elevates FTH and ferritin light-chain (FTL), with co-expression further enhancing FTL levels. Restoration of Dmt1 expression rescues ferritin levels, lysosomal activity, and NICD1 degradation while reducing oxidative stress and lipid peroxidation. Notably, Dmt1 deficiency decreases NICD1 binding to RBP-Jκ/CSL and its recruitment to Notch target gene promoters Hes1 and Hey1. Collectively, our findings demonstrate that Dmt1 regulates lysosomal function through iron homeostasis and that lysosomal dysfunction from Dmt1 loss impairs NICD1 degradation and disrupts Notch signaling, linking cellular iron metabolism and Notch pathway activity.