STING is a direct innate immune sensor of cyclic-di-GMP. This study demonstrates that STING, a known signaling adaptor in the interferon response to cytosolic DNA, also directly binds cyclic-di-GMP (c-di-GMP) and c-di-AMP, two cyclic dinucleotides produced by bacteria. STING binds directly to radiolabeled c-di-GMP, and this binding is competed by unlabelled cyclic dinucleotides but not by other nucleotides or nucleic acids. Mutations in STING that selectively affect the response to cyclic dinucleotides do not affect the response to DNA, indicating that STING functions as a direct sensor of cyclic dinucleotides in addition to its established role in the interferon response to DNA. Cyclic dinucleotides, such as c-di-GMP and c-di-AMP, are conserved signaling molecules in bacteria and have shown promise as novel vaccine adjuvants and immunotherapeutics. However, the mammalian sensor of cyclic dinucleotides has not been identified. This study shows that STING is the mammalian sensor of cyclic dinucleotides. STING binds c-di-GMP directly and specifically, and this binding is not competed by other nucleotides or nucleic acids. STING's C-terminal domain is involved in binding cyclic dinucleotides, and mutations in this domain affect the response to cyclic dinucleotides but not to DNA. STING is essential for the interferon response to cyclic dinucleotides, and its expression is sufficient to restore responsiveness of HEK293T cells to cyclic dinucleotides but not to DNA. STING binds c-di-GMP directly and specifically, and this binding is not competed by other nucleotides or nucleic acids. STING's C-terminal domain is involved in binding cyclic dinucleotides, and mutations in this domain affect the response to cyclic dinucleotides but not to DNA. STING functions as a direct sensor of cyclic dinucleotides and as a signaling adaptor in the interferon response to DNA. The study provides insight into the mechanism by which cyclic dinucleotides are sensed by the innate immune system.STING is a direct innate immune sensor of cyclic-di-GMP. This study demonstrates that STING, a known signaling adaptor in the interferon response to cytosolic DNA, also directly binds cyclic-di-GMP (c-di-GMP) and c-di-AMP, two cyclic dinucleotides produced by bacteria. STING binds directly to radiolabeled c-di-GMP, and this binding is competed by unlabelled cyclic dinucleotides but not by other nucleotides or nucleic acids. Mutations in STING that selectively affect the response to cyclic dinucleotides do not affect the response to DNA, indicating that STING functions as a direct sensor of cyclic dinucleotides in addition to its established role in the interferon response to DNA. Cyclic dinucleotides, such as c-di-GMP and c-di-AMP, are conserved signaling molecules in bacteria and have shown promise as novel vaccine adjuvants and immunotherapeutics. However, the mammalian sensor of cyclic dinucleotides has not been identified. This study shows that STING is the mammalian sensor of cyclic dinucleotides. STING binds c-di-GMP directly and specifically, and this binding is not competed by other nucleotides or nucleic acids. STING's C-terminal domain is involved in binding cyclic dinucleotides, and mutations in this domain affect the response to cyclic dinucleotides but not to DNA. STING is essential for the interferon response to cyclic dinucleotides, and its expression is sufficient to restore responsiveness of HEK293T cells to cyclic dinucleotides but not to DNA. STING binds c-di-GMP directly and specifically, and this binding is not competed by other nucleotides or nucleic acids. STING's C-terminal domain is involved in binding cyclic dinucleotides, and mutations in this domain affect the response to cyclic dinucleotides but not to DNA. STING functions as a direct sensor of cyclic dinucleotides and as a signaling adaptor in the interferon response to DNA. The study provides insight into the mechanism by which cyclic dinucleotides are sensed by the innate immune system.