Chronic inflammatory T-cell-mediated diseases such as inflammatory bowel disease (IBD) are often treated with immunosuppressants including corticosteroids. In addition to the intended T-cell suppression, these farmacons give rise to many side effects. Recently, immunosuppressive phospholipids have been proposed as less-toxic alternatives. We aimed to investigate the immunoregulatory capacities of the naturally occurring phospholipid phosphatidylinositol (PI). Systemic PI treatment dramatically reduced disease severity and intestinal inflammation in murine 2,4,6-trinitrobenzene sulfonic acid (TNBS) colitis. Moreover, PI treatment inhibited the inflammatory T-cell response in these mice, as T cells derived from colon-draining LN of PI-treated mice secreted less IL-17 and IFN-gamma upon polyclonal restimulation when compared to those of saline-treated mice. Further characterization of the suppressive capacity of PI revealed that the phospholipid suppressed Th cell differentiation in vitro irrespective of their cytokine profile by inhibiting proliferation and IL-2 release. In particular, PI diminished IL-2 mRNA expression and inhibited ERK1-, ERK-2-, p38- and JNK-phosphorylation. Crucially, PI did not ablate Treg differentiation or the antigen-presenting capacity of DCs in vitro. These data validate PI as a pluripotent inhibitor that can be applied mucosally as well as systemically. Its compelling functions render PI a promising novel physiological immune suppressant.