2′3′-cyclic guanosine monophosphate–adenosine monophosphate (cGAMP) is the endogenous agonist of STING; as such, cGAMP has powerful immunostimulatory activity, due to its capacity to stimulate type I interferon-mediated immunity. Recent evidence indicates that cancer cells, under certain conditions, can release cGAMP extracellularly, a phenomenon currently considered important for therapeutic responses and tumor rejection. Nonetheless, the mechanisms that regulate cGAMP activity in the extracellular environment are still largely unexplored. In this work, we collected evidence demonstrating that CD38 glycohydrolase can inhibit extracellular cGAMP activity through its direct binding. We firstly used different cell lines and clinical samples to demonstrate a link between CD38 and extracellular cGAMP activity; we then performed extensive in silico molecular modeling and cell-free biochemical assays to show a direct interaction between the catalytic pocket of CD38 and cGAMP. Altogether, our findings expand the current knowledge about the regulation of cGAMP activity. Display omitted •Doxorubicin increases the release of extracellular cGAMP by myeloma cells•CD38 inhibition causes the accumulation of extracellular cGAMP•CD38 inhibits extracellular cGAMP activity through its direct binding Biochemical mechanism; Molecular biology; Components of the immune system; Cell biology; Cancer; In silico biology