Cancer causes over 10 million deaths annually worldwide and remains a significant global health challenge. This study investigated advanced immunotherapy strategies, focusing on mRNA vaccines that target tumor-specific antigens to activate the immune system. We developed a novel mRNA vaccine using O,O′-dimyristyl-N-lysyl aspartate (DMKD) to improve stability and phosphatidylserine (PS) to enhance antigen presentation to immune cells. This vaccine, containing melanoma-associated antigen A3 (MAGE-A3) mRNA encapsulated within lipid nanoparticles (LNPs), was evaluated for its therapeutic potential against colorectal cancer. Our findings demonstrated that MAGE-A3 mRNA-containing DMKD-PS LNPs significantly reduced tumor size and weight, effectively combating metastatic cancer. The vaccine elicited a robust immune response, increasing specific immunoglobulin and cytokine levels without causing histotoxicity in major organs. These results confirm that the DMKD-PS-based MAGE-A3 mRNA vaccine holds promise for cancer prevention and treatment.