Annexin-1 is a well-known endogenous anti-inflammatory protein that modulates the activation of cells of the innate immune system such as neutrophils and macrophages. We have recently reported a positive role for the exogenous protein on T cell differentiation, however, whether such a role holds true for the endogenous protein has yet to be determined. This aspect has been investigated here finding that Annexin-1-deficient T cells display an impaired activation and proliferation in response to anti-CD3 plus anti-CD28 stimulation. Furthermore, differentiation of T cells from Annexin-1-deficient mice in Th0/Th1/Th2 or Th17 skewing conditions demonstrated an increased Th2 phenotype compared to cells from control littermates. Similar results were obtained when we analyzed the Th1/Th2 profile of lymph node cells obtained from mice immunized with keyhole limpet hemocyanin or the inflammatory infiltrate in mouse model of allergic inflammation. These results demonstrate a novel modulatory role of endogenous Annexin-1 in TCR signaling and T cell differentiation and suggest this protein might play a dual and complementary role in the innate and adaptive immune response.