Autoimmune disorders such as lupus, rheumatoid arthritis, and Crohn's disease affect a significant portion of the global population. They stem from the presence of self-reactive lymphocytes that trigger inflammation and subsequent organ-specific damage. Conventional therapies like NSAIDs and immunosuppressants lack specificity and efficacy, making the exploration of adoptive cell therapies a safer alternative. Regulatory T cells (Tregs) have garnered attention due to their immunosuppressive potential. However, using Tregs for therapeutic purposes remains a great challenge due to their low abundance in the T cell population and lack of unique surface markers making it difficult to identify, isolate, and harvest them for use. Attempts at engineering Foxp3 T cells through intracellular delivery of Foxp3 protein have been made, but were largely unsuccessful due to inefficient uptake of the Foxp3 proteins. To address this challenge, we employed ionizable lipid nanoparticles (LNPs) to deliver Foxp3 mRNA to CD4+ T cells with the goal of inducing expression of the Foxp3 marker in these T cells. We successfully engineered CD4+ T cells into Foxp3-T (FP3T) cells that transiently exhibited an immunosuppressive phenotype and functionally suppressed the proliferation of T effector cells. This study demonstrates the promise of an LNP platform for engineering immunosuppressive T cells with potential applications in autoimmunity therapies.