Recent studies indicated that antimicrobial peptides (AMPs) play multiple roles in both innate and adaptive immune functions. The penaeidin of tiger shrimp Penaeus monodon shows an antimicrobial activity against Gram-positive bacteria and filamentous fungi. To study immunomodulation functions of the penaeidin, we transfected shrimp hemocytes in primary culture with penaeidin-specific small interfering RNA (siRNA-3) and observed a concomitant 20% reduction in adhesive hemocytes compared with mock-transfected cells. The addition of biosynthesized or chemically synthesized penaeidin or penaeidin proline-rich domain (PRD) to the culture medium of penaeidin knock-down hemocytes led to a full recovery in the number of adhesive hemocytes. The effect of penaeidin knock-down on the expression of tiger shrimp cell adhesion-associated molecules was examined using real-time Q-PCR. Results demonstrated 91% and 64% decreases in the expression of integrin-beta and collagen, respectively, and a 396% increase in the expression of collagenase. The addition of chemically synthesized penaeidin after penaeidin knock-down hemocytes normalized the expression of these genes. The addition of the integrin-beta ligand competitor RGDS to mock-transfected hemocytes decreased the number of adhesive hemocytes similar to penaeidin knock-down. In conclusion, penaeidin possesses an integrin-beta-mediated cytokine feature that promotes shrimp granulocyte and semi-granulocyte adhesion. This is the first report about functional shrimp cytokine.