AbstractChronic 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‐γ 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.