Mating in insects commonly induces a profound change in the physiology and behavior of the female that serves to secure numerous and viable offspring, and to ensure paternity for the male by reducing receptivity of the female to further mating attempts. Here, we set out to characterize the post-mating response (PMR) in a pest insect, the brown planthopper Nilaparvata lugens and to identify a functional analog of sex peptide and/or other seminal fluid factors that contribute to the PMR in Drosophila. We find that N. lugens display a distinct PMR that lasts for about 4 days and includes a change in female behavior with decreased receptivity to males and increased oviposition. Extract from male accessory glands (MAG) injected into virgin females triggers a similar PMR, lasting about 24h. Since sex peptide does not exist in N. lugens, we screened for candidate mediators by performing a transcriptional and proteomics analysis of MAG extract. We identified a novel 51 amino acid peptide present only in the MAG and not in female N. lugens. This peptide, that we designate maccessin (macc), affects the female PMR. Females mated by males with macc knockdown display receptivity to wild type males in a second mating, which does not occur in controls. However, oviposition is not affected. Injection of recombinant macc reduces female receptivity, with no effect on oviposition. Thus, macc is an important seminal fluid peptide that affects the PMR of N. lugens. Our analysis suggests that the gene encoding the macc precursor is restricted to species closely related to N. lugens.