To investigate mycothiol peroxidase (MPx) of Corynebacterium glutamicum that is a novel CysGPx family peroxidase using both the mycoredoxin and thioredoxin reducing systems as proton donors for peroxide detoxification and may be involved in the relief of acid stress. A Delta mpx mutant exhibited significantly decreased resistance to acid stress and markedly increased accumulation of reactive oxygen species (ROS) and protein carbonylation levels in vivo. Over-expression of mpx increased the resistance of C. glutamicum to acid stress by reducing ROS accumulation. The stress-responsive extracytoplasmic function-sigma (ECF-sigma) factor, SigH, mediated acid-induced expression of mpx in the wild-type under acid conditions, which in turn directly contributed to tolerance to acid stress. MPx is essential for combating acid stress by reducing intracellular ROS levels induced by acid stress in C. glutamicum, which adds a new dimension to the general physiological functions of CysGPx.