Group A streptococcus (GAS) is an important human pathogen that causes a wide spectrum of diseases, including necrotizing fasciitis and streptococcal toxic shock syndrome. Dextromethorphan (DM), an antitussive drug, has been demonstrated to efficiently reduce inflammatory responses thereby contributes to an increased survival rate of GAS-infected mice. However, the anti-inflammatory mechanisms underlying DM treatment in GAS infection remain unclear. DM is known to exert neuroprotective effects through a NADPH oxidase-dependent regulation. In the present study, membrane translocation of NADPH oxidase subunit p47phox and subsequent reactive oxygen species (ROS) generation induced by GAS infection were significantly inhibited via DM treatment in RAW264.7 murine macrophage cells. Further determination of proinflammatory mediators revealed that DM effectively suppressed inducible nitric oxide synthase (iNOS) expression, NO, TNF-α and IL-6 generation in GAS-infected RAW264.7 cells and in air-pouch-infiltrating cells from GAS/DM co-administrated mice as well. GAS infection caused AKT dephosphorylation, glycogen synthase kinase-3β (GSK-3β) activation, and subsequent NF-κB nuclear translocation, which were also markedly inhibited by the presence of DM and NADPH oxidase inhibitor, diphenylene iodonium. These results suggest that DM attenuates GAS infection-induced overactive inflammation by inhibiting NADPH oxidase-mediated ROS production that leads to down-regulation of the Akt/GSK-3β/NF-κB/NO signaling pathway. Therefore, DM may provide a substantial therapeutic application against GAS-septic shock.