Interleukin (IL)-11 is a multifunctional cytokine belonging to the IL-6 family, which plays essential roles in immune response. However, much less is known about the immunological functions of IL-11 in teleost. In this study, we investigated the immune properties of a teleost IL-11 homologue (CsIL-11) from tongue sole Cynoglossus semilaevis. CsIL-11 possesses four conserved α-helices and conserved CsIL-11 receptor binding residues L86 and R187, and shares 23.3%-80.1% identities with other IL-11 homologues. CsIL-11 expression was constitutive in tissues, with most abundant in blood and least abundant in spleen, and upregulated by bacterial challenge in blood, spleen, and head kidney. Recombinant CsIL-11 (rCsIL-11) in the native form of monomer, could bind to peripheral blood leukocytes (PBLs) membrane and enhance the activation and phagocytosis of PBLs. When administered in vivo, rCsIL-11 could markedly promote the host to defend against microbial infection. Overall, our findings show that CsIL-11 plays a pivotal role in regulating PBLs phagocytosis and antibacterial immunity.