Bacillus anthracis proliferates to high levels within vertebrate tissues during the pathogenesis of anthrax. This growth is facilitated by the acquisition of nutrient iron from host heme. However, heme acquisition can lead to the accumulation of toxic amounts of heme within B. anthracis. Here, we show that B. anthracis resists heme toxicity by sensing heme through the HssRS two-component system, which regulates expression of the heme-detoxifying transporter HrtAB. In addition, we demonstrate that B. anthracis exhibits elevated HssRS function compared to its evolutionary relative S. aureus. Elevated heme sensing is likely required by B. anthracis due to the significant heme sensitivity exhibited by members of the genus Bacilli. We also demonstrate that B. anthracis depends on conserved residues within the previously uncharacterized sensing domain of the histidine kinase HssS for HssS function. Finally, we show that the heme- and HssRS-regulated hrtAB promoter is activated in a murine model of anthrax. These results demonstrate the evolutionary conservation of heme sensing among multiple Gram-positive bacteria and begin to provide a mechanistic explanation for the heme resistance of B. anthracis. Further, these data suggest that heme stress is experienced by bacterial pathogens during infection.