Agonists of mouse STING (TMEM173) shrink and even cure solid tumors by activating innate immunity; human STING (hSTING) agonists are needed to test this therapeutic hypothesis in humans. The endogenous STING agonist is 2′3′-cGAMP, a second messenger that signals the presence of cytosolic double-stranded DNA. We report activity-guided partial purification and identification of ecto-nucleotide pyrophosphatase/phosphodiesterase (ENPP1) to be the dominant 2′3′-cGAMP hydrolyzing activity in cultured cells. The hydrolysis activity of ENPP1 was confirmed using recombinant protein and was depleted in tissue extracts and plasma from Enpp1^−/− mice. We synthesized a hydrolysis-resistant bisphosphothioate analog of 2′3′-cGAMP (2′3′-cG^sA^sMP) that has similar affinity for hSTING in vitro and is ten times more potent at inducing IFN-β secretion from human THP1 monocytes. Studies in mouse Enpp1^−/− lung fibroblasts indicate that resistance to hydrolysis contributes substantially to its higher potency. 2′3′-cG^sA^sMP is therefore improved over natural 2′3′-cGAMP as a model agonist and has potential as a vaccine adjuvant and cancer therapeutic.