ArsH is widely distributed in bacteria, and its function remains to be characterized. In this study, we investigated the function of ArsH from Synechocystis sp. PCC 6803. The inactivation of arsH by insertion of a kanamycin-resistance gene in Synechocystis sp. PCC 6803 resulted in the decrease of arsenic and chromium accumulation compared with the wild type. ArsH expression in Escherichia coli strain Rosetta increased its resistance to chromate by reducing chromate in the medium and cells to chromium (III). In addition, ArsH in Rosetta conferred resistance to arsenic. The purified Synechocystis ArsH was able to reduce chromate and ferric iron at the expense of NADPH. Nonlinear regression values of K0.5 for chromate and ferric iron were 71.9 ± 17.8 μM and 59.3 ± 13.8 μM, respectively. The expression level of arsH was induced by arsenite and arsenate, but not chromate or ferric iron. Our results suggest that Synechocystis ArsH had no substrate specificities and shared some biochemical properties that other enzymes possessed. ArsH may be involved in coordinating oxidative stress response generated by arsenic.