The parasitic wasp Nasonia vitripennis suppresses host immune mechanisms that include melanization reactions. Melanization is an important immune response of hosts induced by wasp infection and thus its inhibition represents a successful strategy for parasitism. However, the molecular basis associated with such inhibition is largely unknown in N. vitripennis. Here, we report recombinant expression, structural and functional characterization of a Nasonia-derived defensin-like peptide (called nasonin-3) whose recombinant product exerts inhibitory effect on host melanization. The possible role of nasonin-3 in immune suppression is also discussed.