Reactive oxygen species (ROS) are critical for maintaining cellular homeostasis and function. The main source of intracellular ROS depends on the NADPH oxidase (Nox) and plays crucial roles in immune deafens in animal. However, the function of Nox in crustaceans remains unclear. In the present study, a Nox gene from the mud crab (designated as Sp-Nox) was cloned and identified. The full length of Sp-Nox contained an open reading frame of 3939 bp encoding 1312 amino acids, a 5' untranslated region (UTR) of 420 bp, and a 3' UTR of 813 bp. The deduced amino acid sequences of Sp-Nox contained a typical flavin adenine dinucleotide (FAD) binding domain and a nicotinamide adenine dinucleotide (NAD) binding domain. Sp-Nox was widely expressed in all tested tissues, with the highest expression levels in the gill. Sp-Nox expression in hepatopancreas was significantly up-regulated after V. parahaemolyticus infection. In order to know more about the regulation mechanism of Sp-Nox, RNA interference experiment was investigated. Knocking down Sp-Nox in vivo can significantly reduce the production of ROS and the expression levels of antioxidant-related genes (CAT and SOD). Moreover, Sp-Nox interference significantly increased the mortality of mud crabs after V. parahaemolyticus infection. All these results suggested that Sp-Nox played a crucial role in the defense against V. parahaemolyticus infection in crustaceans.