The emergence and spread of bacterial resistance highlights the urgent need for new antibiotics. Organoarsenicals have been used as antimicrobials since Paul Ehrlich’s salvarsan. Recently a soil bacterium was shown to produce the organoarsenical arsinothricin (AST), a mimetic of the herbicidal antibiotic phosphinothricin (PT). We demonstrate that AST is an effective broad-spectrum antibiotic, showing that bacteria have acquired the ability to utilize environmental arsenic to produce a potent antimicrobial. With every new antibiotic, resistance inevitably arises. The arsN gene, widely distributed in ars operons, confers PT resistance. The functional linkage of arsN to arsenic detoxification was unclear. Here we show that arsN selectively confers resistance to AST. Crystal structures of the ArsN N-acetyltransferase shed light on the mechanism of selectivity. These results may lead to development of a new class of antimicrobials and ArsN inhibitors.