Toll-like receptor 8 (TLR8) recognizes viral and bacterial RNA, initiating inflammatory responses that are crucial for innate immunity. Dysregulated TLR8 signaling contributes to autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis, driving chronic inflammation and tissue damage. Therefore, targeting TLR8 has gained attention as a promising therapeutic strategy. We report a novel selective TLR8 antagonist scaffold identified through computational modeling and simulation. In silico-guided rational drug design and synthesis led to potent isoxazole-based compounds that were characterized by structure-activity relationships. The most active compounds inhibited TLR8-mediated signaling in cell lines and primary cells, reduced MyD88 recruitment, suppressed NF-κB- and IRF-dependent signaling, and decreased inflammatory responses. In silico and pharmacological analyses demonstrated competitive binding to the pocket of chemical ligands within the TLR8 dimerization interface. These highly selective and potent TLR8 antagonists possess favorable physicochemical properties, representing potential clinical candidates for TLR8-targeted therapy.