H2S is the third gaseous signaling molecule next to nitric oxide and carbon monoxide, but studies on its physiological functions in bacteria are just emerging. In this paper, we review recent findings regarding endogenous production and physiological functions of H2S in facultative anaerobic bacteria, partly based on our own research on Shewanella oneidensis. There are two principal H2S producing pathways in S. oneidensis:one is through cysteine degradation, and the other is via inorganic sulfur respiration. Endogenous H2S could either benefit mutual growing bacteria by supplying energy and inorganic, or inhibit competing bacteria. Our review attaches particular importance to the role of H2S in bacterial oxidative stress response. On one hand, H2S is able to directly inhibit heme-containing catalase, enhancing killing by H2O2. On the other hand, H2S could activate oxidative response as a signaling molecule, leading to cell protection from the oxidative stress due to elevated expression of H2O2 scavenging and repairing systems. Intriguingly, the dominance of either role is determined by H2S-treating time, that is, inhibition is the immediate response whereas activation of oxidative stress response needs extended treatment. The elucidation of endogenous production and its physiological function of H2S in facultative anaerobic bacteria would improve understanding of biogeochemical sulfur recycling, and facilitate control of infectious bacterial pathogens.