Background: Despite the excellent anti-inflammatory and anti-rheumatic efficacy associated with kirenol generation, the content of kirenol in Siegesbeckea orientalis is quite low. Objective: This study was designed to establish a reliable kirenol production protocol by transformed root cultures of S. orientalis and to investigate the antimicrobial activities of kirenol, hairy root, and S. orientalis. Materials and Methods: Transformed root cultures of S. orientalis were established by the transformation of Agrobacterium rhizogenes A4. Transgenic status of the roots was confirmed by polymerase chain reaction (PCR) using rolB specific primers. The biomass and kirenol accumulation of hairy root clones were assessed using four different culture media: MS, MS/2, B5, and white. The antimicrobial activities of kirenol, hairy root, and S. orientalis were evaluated by the disc diffusion method. Results: The optimum media for kirenol synthesis was MS. The content of kirenol in transformed hairy roots made up about 80% of that observed in natural leaves of S. orientalis (1.6 mg/g dry weight). All tested samples displayed antimicrobial activity against Gram-positive pathogens including Staphylococcus epidermidis, Staphylococcus aureus, and Acinetobacter baumannii, with MIC ranging from 78 to 625 μg/mL. Discussion and Conclusion: The high level of kirenol contents was obtained from hairy roots of S. orientalis. Kirenol was effective against gram-positive bacteria. Interestingly, the extract from hairy roots showed a diverse antimicrobial effect from that of kirenol and S. orientalis.