The cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) signaling axis represents a pivotal immunostimulatory pathway and an attractive pharmacological target in oncology. Its activation within the tumor microenvironment promotes cross-priming of tumor-associated antigens and enhances infiltration of effector T lymphocytes. Owing to its potent antitumor activity, the cGAS-STING pathway offers significant promise for the development of cancer vaccines, immunotherapeutic strategies such as antibody-drug conjugates, and interventions against virus-driven malignancies. However, translating preclinical findings to clinical applications has been challenging due to species-specific differences between human and mouse cGAS and STING. A model expressing human STING only has been previously reported and showed to be a valuable tool to investigate the activity of STING agonist in different tumor types. Herein, we report an immunocompetent mouse model expressing both human cGAS and human STING (genO-hcGAS-hSTING). Humanization of cGAS and STING did not alter immune cell composition in the spleen, blood and bone marrow when compared to WT mice. Expression of human cGAS has been confirmed in brain and lung by Western Blot, while hSTING expression was confirmed in immune cells in the spleen and blood by flow cytometry. Functionality of cGAS was investigated through activation of splenocytes with the non-human-specific cGAS agonist G3-YSD, which induced IFN-alpha, IFN-gamma, and TNF-alpha secretion. Similarly, functionality of STING has also been investigated by activating splenocytes with diABZi and DMXAA. While diABZi activates both human and mouse STING, DMXAA activates specifically mouse STING. Indeed, diABZi induces the secretion of IFN-beta, CXCL10, and IL-6 in cells from both WT genO-hcGAS-hSTING mice, wherein DMXAA induced cytokine secretion in WT only. These data indicate that human STING expressed in genO-hcGAS-hSTING mice is functional and mouse STING is no longer expressed in these mice. These humanized models provide valuable tools for assessing the efficacy and specificity of compounds targeting human cGAS and STING pathways, potentially streamlining drug discovery and enhancing our understanding of innate immune responses in human health and disease.