The mechanism of sex determination remains poorly understood in hemimetabolous insects. Here, in the brown planthopper (BPH), Nilaparvata lugens, a hemipteran rice pest, we identified a feminizing switch or a female determiner (Nlfmd) that encodes a serine/arginine-rich protein. Knockdown of Nlfmd in female nymphs resulted in masculinization of both the somatic morphology and doublesex splicing. The female-specific isoform of Nlfmd, Nlfmd-F, is maternally deposited and zygotically transcribed. Depletion of Nlfmd by maternal RNAi or CRISPR-Cas9 resulted in female-specific embryonic lethality. Knockdown of an hnRNP40 family gene named female determiner 2 (Nlfmd2) also conferred masculinization. In vitro experiments showed that an Nlfmd2 isoform, NlFMD2340, bound the RAAGAA repeat motif in the Nldsx pre-mRNA and formed a protein complex with NlFMD-F to modulate Nldsx splicing, suggesting that NlFMD2 may function as an RNA binding partner of the feminizing switch NlFMD. Our results provide novel insights into the diverse mechanisms of insect sex determination.