All heavy metals can lead to toxicity and oxidative stress when taken up in excessive amounts, imposing a serious threat to the environment and human health. Expression of genes for resistance to heavy metals and metalloids is frequently transcriptionally regulated by the toxic ions themselves. Arsenic is a ubiquitous, naturally occurring toxic metalloid widely distributed in soil and groundwater. Microbes biotransform both pentavalent (As(V)) and trivalet (As(III)) arsenic into more the toxic methylated metabolites methylarsenite acid (MAs(III)) and dimethylarsenite (DMAs(III)). Environmental arsenic is sensed by members of the ArsR/SmtB family of metalloregulatory proteins. The arsR gene is autoregulated and is typically part of an operon that contains other ars genes involved in arsenic detoxification. To date, every identified ArsR is regulated by inorganic As(III). Here we described a novel ArsR from Shewanella putrefaciens that is specific for MAs(III) and does not respond to As(III). SpArsR has three conversed cysteine: Cys83, Cys101 and Cys102. Substitutions of Cys83 with serine has no effect for ArsR activity. However, mutation of C101S and C102S mutants lost MAs(III) binding affinity, which indicates that these two cysteines are required for MAs(III) binding. SpArsR can be converted into an As(III)-responsive repressor by introduction of an additional cysteine that allows for 3-coordinate As(III) binding. Our results indicate that SpArsR evolved selectivity for MAs(III) over As(III) in order to control expression of genes for MAs(III) detoxification.