The immune deficiency (IMD) signal pathway mediates innate immunity against Gram-negative bacteria in crustaceans. In the present study, an IMD homolog (MnIMD) from the oriental river prawn, Macrobrachium nipponense was identified. The full-length cDNA of MnIMD was 782bp with an open reading frame of 549 bp that encodes a putative protein of 182 amino acids including a death domain at the C-terminus. Multiple alignment analysis showed that IMDs in prawn M. nipponense and other crustaceans shared high similarity. The recombinant protein of MnIMD was expressed and purified for further functional analyses. Western blot analysis indicated that MnIMD was present in many tissues, but with the highest level in the gills, which was consistent with the qRT-PCR results. After Vibrio parahaemolyticus challenge, MnIMD was significantly induced in gills. RNA interference analysis showed that the IMD pathway was involved in regulating the expression of different antimicrobial peptide (AMP) genes, including Cru4 and Cru6. These results are helpful in promoting research on the innate immunity in M. nipponense.