Background: 5-aminolevulinic acid-based photodynamic therapy (ALA–PDT) has emerged as a promising cancer treatment owing to its selectivity and minimal invasiveness. However, certain cancer types exhibit resistance to ALA–PDT, partly due to insufficient protoporphyrin Ⅸ (PpⅨ) accumulation. This study aimed to enhance PpⅨ accumulation by modulating iron metabolism. Methods: Two cancer cell lines were used in this study, TMK-1 and MCF-7. Divalent metal transporter 1 (DMT1) was knocked down genetically, and the iron chelator deferoxamine (DFO) was used to downregulate ferrochelatase (FECH). The cellular levels of PpⅨ, reactive oxygen species, Fe2+, and cell viability were determined to evaluate the efficacy of ALA–PDT. Results: Knocking down DMT1 increased PpⅨ accumulation in TMK-1 cells but did not contribute to it in MCF-7 cells. DFO enhanced PpⅨ accumulation in both cell lines, primarily by removing excess iron ions and inhibiting FECH. Conclusion: This study offers a novel strategy for improving the efficacy of ALA–PDT through the modulation of iron metabolism, achieved by targeting DMT1 and utilizing the iron chelator DFO. Additionally, DFO exhibited a dual mechanism, depending on cellular iron homeostasis.