The activation of the stimulator of interferon genes (STING) pathway triggers the release of type I interferons that can potentiate the host immune response against tumors. STING agonism is therefore a promising strategy for the development of cancer immunotherapy; however, sensitive tools and assays for the discovery of STING modulators are currently limited. Here, we develop and characterize a STING ligand sensor, FiSL, to detect STING ligands in vitro. Utilizing FiSL, we identify honokiol, a natural compound derived from Magnolia species, as an orally available STING agonist from a bioactive compound library. Functional studies reveal that honokiol exerts antitumor activity in a STING-dependent manner. Moreover, in STING-humanized mouse tumor models, honokiol enhances the efficacy of anti-PD-(L)1 immunotherapy. Collectively, we have developed FiSL as a tool for high-throughput screening of STING ligands and revealed honokiol as a STING agonist that can be harnessed to treat human cancer.