Cytosolic detection of microbial products is essential for the initiation of an innate immune response against intracellular pathogens such as Mycobacterium tuberculosis (Mtb). During Mtb infection of macrophages, activation of cytosolic surveillance pathways is dependent on the mycobacterial ESX-1 secretion system and leads to type I interferon (IFN) and interleukin-1 beta (IL-1 beta) production. Whereas the inflammasome regulates IL-1 beta secretion, the receptor(s) responsible for the activation of type I IFNs has remained elusive. We demonstrate that the cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS) is essential for initiating an IFN response to Mtb infection. cGAS associates with Mtb DNA in the cytosol to stimulate cyclic GAMP (cGAMP) synthesis. Notably, activation of cGAS-dependent cytosolic host responses can be uncoupled from inflammasome activation by modulating the secretion of ESX-1 substrates. Our findings identify cGAS as an innate sensor of Mtb and provide insight into how ESX-1 controls the activation of specific intracellular recognition pathways.