Superoxide dismutase (SOD) is one of the main antioxidant enzymes, and its expression level may be significantly affected by environmental and biological stimuli. Therefore, SOD is considered as an important component of invertebrate innate immunity, though the mechanism of its contribution to immune responses is rarely reported. In this study, we cloned the cytoplasmic manganese SOD of Macrobrachium rosenbergii (MrcMnSOD), prepared its polyclonal antibodies, and analyzed its expression pattern under Aeromonas hydrophila infection. The results showed that A. hydrophila infection significantly increased the expression of MrcMnSOD both on the transcription level and protein level. In order to explore the possible mechanism of MrcMnSOD involvement in the immune response, further antibacterial assays were conducted. The results showed that MrcMnSOD can significantly inhibit the growth of three gram-negative bacteria (Escherichia coli, Vibrio parahaemolyticus, A. hydrophila) and two gram-positive bacteria (Streptococcus agalactiae, Staphylococcus aureus), and the inhibitory effect is largely irrespective of protein concentration. The present study indicates that MrcMnSOD may participate in the immune response as an immune-related molecule. This study reportes the immunobiological function of MrcMnSOD preliminarily, aiming to lay the relevant foundation for further research on SOD.