Diffuse large B-cell lymphoma (DLBCL) is highly heterogeneous and prone to epigenetic mutations. Non-apoptotic cell death pathways, especially ferroptosis, have become an emerging direction for cancer treatment. This project was designed to probe into the potential of histone-lysine N-methyltransferase 2D (KMT2D) in modulating DLBCL ferroptosis through epigenetic mechanisms. We employed quantitative real-time polymerase chain reaction (qRT-PCR) and western blot (WB) to detect the expression level of KMT2D, CCK-8 assay to measure cell viability, flow cytometry to assess cell cycle and apoptosis, and fluorescent probes to analyze lipid reactive oxygen species levels. 4-hydroxynonaldehyde (4-HNE) and malondialdehyde were detected by WB and corresponding kits to determine the degree of intracellular ferroptosis. The level of H3K4Me1 was determined by using WB and immunofluorescence. Furthermore, we verified the transcriptional regulatory relationship between KMT2D and SMG1 through bioinformatics analysis, chromatin immunoprecipitation assay. We also applied WB to assess the activation of the AKT-mTOR pathway. Finally, the in vitro experimental results were validated by qRT-PCR, WB, immunohistochemistry, and fluorescent probe detection in a xenograft tumor model constructed in BALB/c nude mice. Overexpression of KMT2D considerably repressed the malignant behavior of DLBCL and triggered ferroptosis in cells. KMT2D was able to bind directly to the promoter region of the SMG1 gene and induce the transcriptional antagonistic mTOR pathway of SMG1 through H3K4Me1, thereby inducing ferroptosis in DLBCL cells. These findings demonstrate that KMT2D reinforces ferroptosis in DLBCL by antagonizing SMG1-mediated mTOR signaling, identifying it as a novel therapeutic target.