The endogenous production of hydrogen sulfide (H2S) and its physiological functions, including membrane hyperpolarization and smooth muscle cell relaxation, position this gas well in the family of gasotransmitters together with nitric oxide (NO) and carbon monoxide (CO). In this study, we demonstrate that H2S at physiologically relevant concentrations induced apoptosis of human aorta smooth muscle cells (HASMCs). Exposure of HASMCs to H2S did not induce necrosis as verified with Trypan blue exclusion and LDH release analysis. After inhibiting endogenous H2S production, exogenous H2S induced much more significant apoptosis, which was not altered by the presence of albumin or glutathione. H2S treatment increased the activities of ERK and p38 mitogen-activated protein kinase (MAPK), but not c-Jun N-terminal kinase activity. Suppression of extracellular signal-regulated kinase (ERK) activity, but not of p38 activity, inhibited the H2S-induced apoptosis of HASMCs. The activation of ERK by H2S in HASMCs was accompanied by increased caspase-3 activity. Inhibition of caspase-3 by AC-DEVD-CHO attenuated the H2S-induced cell apoptosis. Inhibition of ERK by U0126 decreased caspase-3 activity, whereas AC-DEVD-CHO did not alter ERK activity. In conclusion, exogenous H2S induces apoptosis of HASMCs, which is significantly affected by the endogenous H2S level. Of the three investigated MAPKs, only ERK played an active role in mediating H2S-induced apoptosis of HASMCs by activating caspase-3. These findings may help reveal novel mechanisms for many diseases linked to H2S-related abnormal cellular proliferation and apoptosis.