Chemoresistance presents a critical challenge in breast cancer treatment. Here, we report that transient receptor potential melastatin 4 (TRPM4) plays a role in modulating doxorubicin (ADR) resistance in breast cancer cells. TRPM4 expression was significantly upregulated at both the mRNA and protein levels in MCF-7/ADR cells, a human breast cancer cell line resistant to the chemotherapy drug ADR. Pharmacological inhibition or knockdown of TRPM4 restored ADR sensitivity, while its overexpression in non-resistant MCF-7 cells diminished drug response, confirming the regulatory role of TRPM4 in resistance mechanisms. Western blot analyses confirmed that elevated TRPM4 expression drives P-glycoprotein (P-gp) upregulation in both MCF-7/ADR and KBv200 cells (KB vinblastine 200 resistant cell line), as well as in Huh7 (human hepatocellular carcinoma cell line) and HCT116 (human colorectal cancer cell line). In addition, we demonstrate that TRPM4 inhibition suppresses the level of NF-κB, a pivotal transcription factor regulating P-gp expression. Furthermore, we found that TRPM4-mediated cellular swelling, rather than membrane depolarization, is the primary driver of P-gp overexpression. Drug-resistant MCF-7/ADR cells exhibited significantly larger cell sizes compared to non-resistant MCF-7 cells, and this effect was reversed following TRPM4 inhibition. The swelling was induced by hypotonic stress rather than changes in membrane potential, further confirming the role of TRPM4 in P-gp regulation through volume changes. Analysis of the TCGA (The Cancer Genome Atlas) database revealed that elevated TRPM4 expression correlates with reduced patient survival, suggesting that TRPM4 plays a role in both drug resistance and tumor progression. Our findings provide new insights into the role of TRPM4 in resistance mechanisms and propose that targeting TRPM4 could represent an innovative therapeutic strategy to overcome chemoresistance and enhance drug efficacy in breast cancer.